Reactive filter in Leaflet / Shiny producing incorrect results

Question

I'm making a map using Leaflet and Shiny, but have a problem when it comes to the reactive section of my code.

I would like to use a dropdown filter to select just the points within a particular area. Currently, the following code filters points down to the correct area, but - weirdly - they're not the points that should be there.

For example, if I select Area 1, then only points in Area 1 are shown, but they're points from all over the map that shouldn't actually be in Area 1.

I've identified what I think is the problematic section below.

library(leaflet)
library(shiny)
library(dplyr)

dairypoints <- read.csv("Data dairies.csv")
rdmareas <- readOGR("The New Dairy Areas.shp")

ui <- fluidPage(
  titlePanel("Search Dairy Producers in the UK by Area"),
  selectInput("dropdown", "Select Area", choices = c(dairypoints$area), multiple = FALSE),
  leafletOutput("dairypointsmap", height = 1000, width= 1000)
)

pal <- colorFactor(c("blue", "black"), domain = c("Dairy Farm", "RDM Farm"))

#I think it's this section (next 3 lines) that's causing the problem
server <- function(input, output){
  filtered <- reactive({
    dairypoints %>% filter(dairypoints$area==input$dropdown)
  })

  output$dairypointsmap <- renderLeaflet({
    dairypointsmap <- leaflet(filtered()) %>%
      setView(lng = -2.916976, lat = 54.815864, zoom = 6) %>%
      addTiles() %>%
      addLegend("topright", pal = pal, values = ~Type) %>%
      addPolygons(data = rdmareas, 
                  stroke = TRUE, fillOpacity = 0, 
                  smoothFactor = 0.1, color = "black", opacity = 1, weight = 1) %>%
      addCircleMarkers(
        radius = 4,
        color = ~pal(dairypoints$Type),
        stroke = FALSE, fillOpacity = 1,
        popup = paste("Name", dairypoints$Name, "<br>",
                      "Producer ID", dairypoints$ProducerID, "<br>",
                      "Area Number", dairypoints$area, "<br>",
                      "Post Code", dairypoints$Postcode, "<br>",
                      "CPH", dairypoints$CPH, "<br>",
                      "Farm Type", dairypoints$Type))
  })
  options(shiny.sanitize.errors = FALSE)
}

shinyApp(ui = ui, server = server)