Weird problem with `renderDataTable` and `renderUI` with different orders - r

Question

I'm working on a shiny app and trying to use renderDataTable and renderUI on the same page. The following codes work fine. But when I switch the order of 2. render select input and 3. render data table, it seems input$selected_variable cannot reflect what is selected on the shiny app. Neither the print(input$selected_variable) can print the selected value in the terminal, nor the textOutput can show the selected value on the app interface. Does anyone know why this may happen? Another follow-up question is that if I remove filter = 'top' in the renderDataTable, this issue won't occur either no matter which order is applied. Does this problem has anything to do with the filter = 'top'? (This filter = 'top' option in dataTable seemed also cause a few other weird behaviors when I tried out some other shiny app designs.)
if (interactive()) {
library(shiny)
library(DT)
shinyApp(
### ui
ui = fluidPage(
fileInput('input_data_file',
label = "Load input data file(s)",
multiple = TRUE,
buttonLabel = "Browse or Drop...",
placeholder = "(multiple) csv file(s)"),
dataTableOutput('input_table'),
uiOutput('params'),
verbatimTextOutput("text", placeholder = T),
plotlyOutput('plot')
),
### server
server = function(input, output) {
### values placeholder
values <- reactiveValues()
values$data <- NULL
### 1. upload data file
observeEvent(input$input_data_file, {
req(input$input_data_file)
file <- input$input_data_file
ext <- tools::file_ext(file$datapath)
req(file)
validate(need(ext == "csv", "Please upload a csv file"))
values$data <- read.csv(file$datapath, stringsAsFactors = FALSE)
})
### 2. render select input
output$params <- renderUI({
fluidRow(column(6, selectInput("selected_variable", "Variable:",
choices = setdiff(names(values$data), 'CustomerID')
)))
})
### 3. render data table
output$input_table <- DT::renderDataTable(
values$data,
filter = 'top', # removing this line can also solve the issue
options = list(scrollX=TRUE, scrollCollapse=TRUE, stateSave = TRUE, search = list(regex = TRUE)),
server = FALSE)
### 4. render text output
observe({
#req(input$selected_variable)
print(input$selected_variable)
output$text <-renderText({input$selected_variable})
})
}
)
}