That is very strange. Sorry it hasn’t worked for you.
I think you’ll want to try the built-in LS plugin described in the blogpost I shared (it’s also in the second half of the wiki page on vim at the LanguageServer.jl repo); I believe the way that completion is implemented there is a bit friendlier to how julia-vim does L2U-substitution.
If you’d like to keep trying with coc, send me a PM and I can send my init.vim.