Since Mason uses npm to install the language server, it tries to use the latest v0.4.2 of the NPM package.
I'm not sure how the process of releasing/pushing into NPM works in this repository, but it seem that the changes that was added in #63 is not present in the v0.4.2 of the NPM package.
The code looks like this in the code tab in NPM.
const triggerCharacters = [
">",
")",
"]",
"}",
"@",
"*",
"$",
"+",
// and so on ...
Obviously this can be fixed by manually adding the ! character in the emmet-ls in your $HOME/home/lr/.local/share/nvim/mason/bin/emmet-ls file.
But still would be nice if the NPM package was updated to the latest master commit.