vim-pandoc bibliography completion works only when writing a new file with GVim

Question

I have installed vim-pandoc, as suggested here. When I open Vim in the terminal I get only 'Omni completion (^O^N^P) Pattern not found' errors. echo pandoc#bibliographies#Find_Bibliographies() returns a list of biblatex files, as expected.

One very strange thing is that if I open GVim and write a new .md file, completion works. But not if I then open the same file either from the terminal or with GVim.

I raised the issue here.

I have Vim 8.2.716 on Ubuntu 20.10.

Here is my .vimrc:

set nocompatible              " be iMproved, required
filetype off                  " required
" set the runtime path to include Vundle and initialize
set rtp+=~/.vim/bundle/Vundle.vim
call vundle#begin()
" alternatively, pass a path where Vundle should install plugins
"call vundle#begin('~/some/path/here')
" let Vundle manage Vundle, required
Plugin 'VundleVim/Vundle.vim'
" pandoc
Plugin 'vim-pandoc/vim-pandoc'
Plugin 'vim-pandoc/vim-pandoc-syntax'
Plugin 'vim-pandoc/vim-pandoc-after'
" better LaTeX conceal, which pandoc will inherit
Plugin 'KeitaNakamura/tex-conceal.vim'
" All of your Plugins must be added before the following line
call vundle#end()            " required
filetype plugin indent on    " required
" To ignore plugin indent changes, instead use:
"filetype plugin on
"
" Brief help
" :PluginList       - lists configured plugins
" :PluginInstall    - installs plugins; append ! to update or just :PluginUpdate
" :PluginSearch foo - searches for foo; append ! to refresh local cache
" :PluginClean      - confirms removal of unused plugins; append ! to auto-approve removal
"
" see :h vundle for more details or wiki for FAQ
" Put your non-Plugin stuff after this line
" UK spelling for Markdown
autocmd FileType markdown setlocal spell spelllang=en_gb
" recognise .md as Markdown
autocmd BufNewFile,BufReadPost *.md set filetype=markdown.pandoc
" case insensitive search
set ignorecase
" tabs are 4 spaces
set expandtab
set shiftwidth=4
set softtabstop=4
" omnicompletion
set omnifunc=syntaxcomplete#Complete
set completeopt=menu,menuone
" conceal settings
set conceallevel=2
let g:tex_conceal="abdgm"
" look for bibliographies
let g:pandoc#biblio#sources="l"
" omnicomplete
set omnifunc=syntaxcomplete#Complete

What could be causing this?