Previous selectizeInput value staying at top of choices when using actionButton

The app below works as expected on startup and when clicking the "Next Vehicle >" actionButton for the first time.

When I press the actionButton a second time, it selects the third vehicle as expected, but when I open the dropdown, the previously selected value "Cadillac Fleetwod" is listed first, when I would expect it to be listed second.

If I press the actionButton a third time, "Chrysler Imperial" is correctly selected, but now "Camaro Z28" is listed first, when I would expect it to be listed third. Note, "Cadillac Fleetwood" and "AMC Javelin" are correct if you ignore the first value.

If I avoid using the actionButton altogether and instead manually click every time, the ordering in the dropdown menu is as expected.

The previous value in input$vehicle is being listed first every time when I use the actionButton. How can I prevent this from happening and have my choices for input$vehicle be listed alphabetically when I use the button regardless of the previous value?

library(shiny)

df <- mtcars
df$vehicle <- rownames(df)
df <- df[order(df$vehicle), c("vehicle", colnames(mtcars))]

ui <- fluidPage(
  sidebarLayout(
    sidebarPanel(
      selectizeInput("vehicle",
                     "Select vehicle",
                     choices=NULL,
                     selected=NULL),
    ),
    mainPanel(
      actionButton("nextbut", "Next Vehicle >") ,
    )
  )
)

server <- function(input, output, session) {
  observe({
      updateSelectizeInput(session,
                           "vehicle",
                           "Select vehicle",
                           df$vehicle,
                           selected=df$vehicle[1],
                           server = T) 
  })
  
  observeEvent(input$nextbut, {
    vehicles <- df$vehicle
    idx <- which(vehicles == input$vehicle)
    if(idx != length(vehicles)){
      updateSelectizeInput(session,
                           "vehicle",
                           label = "Select vehicle",
                           choices = vehicles,
                           vehicles[idx+1],
                           server=T
      )
    }
  })
}

# Run the application 
shinyApp(ui = ui, server = server)

observeEvent(input$nextbut, {
    vehicles <- df$vehicle
    idx <- which(vehicles == input$vehicle)
    if(idx != length(vehicles)){
      updateSelectizeInput(session,
                           "vehicle",
                           label = "Select vehicle",
                           choices = vehicles,
                           vehicles[idx+1],
                           server=T,
                           options = list(
                             sortField = list(
                               field = "text",  
                               direction = "asc"
                             )
                           )
      )
    }
  })