Steps to reproduce

Moving from neovim/neovim#31233

mwe.rst:

================
Document Heading
================

Heading
=======

Sub-heading
-----------

Paragraphs are separated 
by a blank line.

and vim --clean -u vimrc +'set fdm' mwe.rst where vimrc is

set nocompatible
let g:rst_fold_enabled = 1      
let g:markdown_folding = 1      ":set fdm returns manual
"let g:javaScript_fold = 1       ":set fdm returns syntax
filetype plugin on
syntax on

Everything works as expected. Markdown folding doesn't interfere with rst
folding. However, adding any of these lines

let g:javaScript_fold = 1       ":set fdm returns syntax
let g:php_folding = 1       ":set fdm returns syntax

and now rst folding no longer works. I only managed to find php and javascript, but there could be other filetype plugins having the same issue, too.

Expected behaviour

.

Version of Vim

9.1

Environment

Linux

Logs and stack traces

No response