JavaFX ComboBox rowSpan in GridPane not working programatically

Question

I am trying to add programatically a ComboBox to an existing GridPane, called daysGridPane. This ComboBox is supposed to span two rows. The complete setup is a little bit more complex, as the daysGridPane is embedded in another Gridpane, called parentGridPane. Reason is that I need to be able to scroll the content of the daysGridPane, while the rest of the UI remains visible. The trouble is that the added ComboBox does not span the two rows. It only covers the upper cell. When the stage is shown, the first ComboBox below the "Monday"-ComboBox, originates from the fxml definition of the UI in SceneBuilder. I created a second ComboBox in SceneBuilder, spanning two rows, and copied all the setting equivalents to the code. Doesn't work.

On the snapshot you can see the grid-lines. I find it noteworthy, that the added rows are smaller, compared to the rows of the parentGridPane, but I am not sure whether this has to do with the root cause.

Here is the underlying fxml file:

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.geometry.Insets?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.ComboBox?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.ScrollPane?>
<?import javafx.scene.control.Separator?>
<?import javafx.scene.layout.ColumnConstraints?>
<?import javafx.scene.layout.GridPane?>
<?import javafx.scene.layout.RowConstraints?>

<GridPane fx:id="parentGridPane" maxHeight="1.7976931348623157E308" maxWidth="1.7976931348623157E308" stylesheets="@application.css" xmlns="http://javafx.com/javafx/21" xmlns:fx="http://javafx.com/fxml/1" fx:controller="de.levin.ctrl.TrainingPlanEditorCtrl">
   <children>
      <ComboBox fx:id="startHourComboBox" GridPane.columnIndex="1" GridPane.rowIndex="1">
         <GridPane.margin>
            <Insets top="6.0" />
         </GridPane.margin>
      </ComboBox>
      <ComboBox fx:id="startMinutesComboBox" GridPane.columnIndex="3" GridPane.rowIndex="1">
         <GridPane.margin>
            <Insets right="6.0" top="6.0" />
         </GridPane.margin>
      </ComboBox>
      <ComboBox fx:id="endHourComboBox" GridPane.columnIndex="1" GridPane.rowIndex="2">
         <GridPane.margin>
            <Insets bottom="12.0" top="6.0" />
         </GridPane.margin>
      </ComboBox>
      <ComboBox fx:id="endMinutesComboBox" GridPane.columnIndex="3" GridPane.rowIndex="2">
         <GridPane.margin>
            <Insets bottom="12.0" right="6.0" top="6.0" />
         </GridPane.margin>
      </ComboBox>
      <Label text=":" GridPane.columnIndex="2" GridPane.rowIndex="1">
         <GridPane.margin>
            <Insets left="3.0" right="3.0" />
         </GridPane.margin>
      </Label>
      <Label text=":" GridPane.columnIndex="2" GridPane.rowIndex="2">
         <GridPane.margin>
            <Insets left="3.0" right="3.0" />
         </GridPane.margin>
      </Label>
      <Label text="Start" GridPane.rowIndex="1">
         <GridPane.margin>
            <Insets left="12.0" right="6.0" />
         </GridPane.margin>
      </Label>
      <Label text="End" GridPane.rowIndex="2">
         <GridPane.margin>
            <Insets left="12.0" right="6.0" />
         </GridPane.margin>
      </Label>
      <Button fx:id="addDayButton" mnemonicParsing="false" onAction="#addDay" text="+" GridPane.columnIndex="5">
         <GridPane.margin>
            <Insets right="12.0" top="12.0" />
         </GridPane.margin>
      </Button>
      <Button fx:id="addIntervallButton" mnemonicParsing="false" onAction="#addIntervall" text="+" GridPane.columnIndex="1" GridPane.rowIndex="3">
         <GridPane.margin>
            <Insets bottom="12.0" top="12.0" />
         </GridPane.margin>
      </Button>
      <Separator prefWidth="200.0" GridPane.columnSpan="2147483647" GridPane.rowIndex="4">
         <GridPane.margin>
            <Insets bottom="6.0" top="6.0" />
         </GridPane.margin>
      </Separator>
      <Button fx:id="okButton" maxWidth="1.7976931348623157E308" mnemonicParsing="false" onAction="#ok" text="OK" GridPane.columnIndex="1" GridPane.columnSpan="2" GridPane.rowIndex="5">
         <GridPane.margin>
            <Insets bottom="24.0" top="6.0" />
         </GridPane.margin>
      </Button>
      <Button fx:id="cancelButton" mnemonicParsing="false" onAction="#cancel" text="Cancel" GridPane.columnIndex="4" GridPane.rowIndex="5">
         <GridPane.margin>
            <Insets bottom="24.0" top="6.0" />
         </GridPane.margin>
      </Button>
      <ScrollPane fx:id="daysScrollPane" GridPane.columnIndex="4" GridPane.hgrow="ALWAYS" GridPane.rowSpan="3" GridPane.vgrow="ALWAYS">
         <content>
            <GridPane fx:id="daysGridPane" maxWidth="1.7976931348623157E308">
              <columnConstraints>
                <ColumnConstraints />
              </columnConstraints>
              <rowConstraints>
                <RowConstraints minHeight="10.0" vgrow="SOMETIMES" />
                <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="ALWAYS" />
                <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="ALWAYS" />
              </rowConstraints>
               <children>
                  <ComboBox fx:id="dayComboBox">
                     <GridPane.margin>
                        <Insets bottom="6.0" right="12.0" />
                     </GridPane.margin>
                  </ComboBox>
                  <ComboBox fx:id="sessionComboBox" maxHeight="1.7976931348623157E308" GridPane.rowIndex="1" GridPane.rowSpan="2">
                     <GridPane.margin>
                        <Insets right="12.0" />
                     </GridPane.margin>
                  </ComboBox>
               </children>
            </GridPane>
         </content>
         <GridPane.margin>
            <Insets right="12.0" top="12.0" />
         </GridPane.margin>
      </ScrollPane>
   </children>
   <columnConstraints>
      <ColumnConstraints />
      <ColumnConstraints />
      <ColumnConstraints />
      <ColumnConstraints />
      <ColumnConstraints hgrow="ALWAYS" maxWidth="1.7976931348623157E308" minWidth="10.0" />
      <ColumnConstraints />
   </columnConstraints>
   <rowConstraints>
      <RowConstraints minHeight="10.0" />
      <RowConstraints />
      <RowConstraints minHeight="10.0" />
      <RowConstraints minHeight="10.0" />
      <RowConstraints minHeight="10.0" vgrow="NEVER" />
      <RowConstraints minHeight="10.0" vgrow="NEVER" />
   </rowConstraints>
</GridPane>

Here is the code, that is supposed to add the row-spanning ComboBox:

    @FXML
    protected void addIntervall(ActionEvent event) {


        // Find the index for the new column 
        int newRowIndex = parentGridPane.getRowConstraints().size() -3; // add-intervall, ok, cancel button rows

        ...
        
        // extend session daysGridPane
        GridPane.setRowSpan(daysScrollPane, GridPane.getRowSpan(daysScrollPane) + 2);
        
        daysGridPane.setGridLinesVisible(true);
        
        RowConstraints rowC1 = new RowConstraints();    // we need two new rows that are spaned by the new ComboBox
        RowConstraints rowC2 = new RowConstraints();

        rowC1.setVgrow(Priority.ALWAYS);
        rowC1.setMaxHeight(Double.MAX_VALUE);
        rowC1.setFillHeight(true);
        rowC2.setVgrow(Priority.ALWAYS);
        rowC2.setMaxHeight(Double.MAX_VALUE);
        rowC2.setFillHeight(true);
        daysGridPane.getRowConstraints().add(newRowIndex, rowC1 );      
        daysGridPane.getRowConstraints().add(newRowIndex, rowC2);
        
        ComboBox<Session> newSessionCB = new ComboBox<Session>();
        daysGridPane.add(newSessionCB, 0, newRowIndex);
        GridPane.setRowSpan(newSessionCB, 2);
        newSessionCB.setMaxHeight(Double.MAX_VALUE);
        GridPane.setVgrow(newSessionCB, Priority.ALWAYS);
        newSessionCB.setItems(persistance.loadSessions(null));
    }

What am I missing?