Excel Office.js popup menus showing twice in Web view

Question

I have Office.js manifest, adding controls to the cell popup menu. The relevant snippet of the manifest.xml file is:

<ExtensionPoint xsi:type="ContextMenu">
    <OfficeMenu id="ContextMenuCell">
        <Control xsi:type="Menu" id="OffAdd.CLMenu">
            <Label resid="OffAdd.CL.Menu.Label" />
            <Supertip>
                <Title resid="OffAdd.CL.Menu.LabelST"/>
                <Description resid="OffAdd.CL.Menu.Description"/>
            </Supertip>
            <Icon>
                <bt:Image size="16" resid="OffAdd.menuicon_16x16" />
                <bt:Image size="32" resid="OffAdd.menuicon_32x32" />
                <bt:Image size="64" resid="OffAdd.menuicon_64x64" />
                <bt:Image size="80" resid="OffAdd.menuicon_80x80" />
            </Icon>
            <Items>
                <Item id="OffAdd.CL.Comma">
                    <Label resid="OffAdd.CL.Comma.Label" />
                    <Supertip>
                        <Title resid="OffAdd.CL.Comma.LabelST" />
                        <Description resid="OffAdd.CL.Comma.Tooltip" />
                    </Supertip>
                    <Icon>
                      <bt:Image size="16" resid="OffAdd.blank_16x16" />
                      <bt:Image size="32" resid="OffAdd.blank_16x16" />
                      <bt:Image size="80" resid="OffAdd.blank_16x16" />
                    </Icon>
                    <Action xsi:type="ExecuteFunction">
                        <FunctionName>doStuff</FunctionName>
                    </Action>
                </Item>

and repeating for a load of sub-menus. Everything works fine on the desktop, but when running in Excel Web on Office.com, all the sub-menus are showing twice, first in bold, then in plain text and with a big gap between them, so my popup looks something like:

Menu >
SubMenu 1
SubMenu 1

How do I stop it, so I only see my menu items once and without the big gap?

I've tried changing the Label, Supertip to see what it was that was showing up, and it's the label both times.