DT shows table with a slider but not the whole table in dashboard and shiny

I am using R in RStudio, with DT, flexdashboard, and Shiny. Since the recent update, tables in DT::datatable are not showing the whole table in the rendered output.

For example,

datatable(iris, options = list(buttons='excel',
                          autoWidth = TRUE, 
                          pageLength = 50,
                          fixedHeader = TRUE))

will show the whole table if the output is a simple markdown file, but if you use these codes in a dashboard or a shiny, the table automatically got a slider for each page. Check here: https://zsnv7j-quan-cui.shinyapps.io/dashboard/

How can I get the dashboard and shiny to show the complete table (in this case, 50 rows without the slider on the right)? Thank you in advance.