`g:fzf_action` doesn't accept functions

[x-yuri]

• I have fzf 0.23.0 or above
• I have read through https://github.com/junegunn/fzf.vim/blob/master/README.md
• I have read through https://github.com/junegunn/fzf/blob/master/README-VIM.md
• I have read through the manual page of fzf (man fzf)
• I have searched through the existing issues

a.vim:

function! s:build_quickfix_list(lines)
  call setqflist(map(copy(a:lines), '{ "filename": v:val }'))
  copen
  cc
endfunction

let g:fzf_action = {
  \ 'ctrl-q': function('s:build_quickfix_list')}

Not Alpine Linux-specific.

$ docker run --rm -itv $PWD/a.vim:/a.vim alpine
/ # apk add vim bash curl git ripgrep bat
/ # bash
bash-5.1# TERM=xterm-256color
bash-5.1# cd /usr/share/vim
bash-5.1# vim -Nu <(curl https://gist.githubusercontent.com/junegunn/6936bf79fedd3a079aeb1dd2f3c81ef5/raw)
:so /a.vim
:Rg
ctrl-q
:copen

The build_quickfix_list solution was suggested here and here. It doesn't work because for some reason non-strings are not accepted.

UPD I've found the change. I don't get the description, but in fzf.vim actions only ever received the first filename. And it's kind of confusing:

#621
#867
#185 (comment)
https://github.com/junegunn/fzf/blob/a74731d/README-VIM.md#examples