I'm looking at this same issue as well. As best I can tell, it appears that a fork of this project has already done the work:

https://github.com/vqv/sublime-beancount

I haven't verified it yet, but here's the individual commit that may fix the above issue:

vqv@77d5723

@joliver @vqv I have tried to use this fork, did a git clone to ~/Library/Application Support/Sublime Text 3/Packages, and also installed PackageDev.
The syntax highlighting works, but the payee autocompletion doesn't.
I use lowercase dash-separated payees, and they work perfectly in Sublime 3, like this:

2021-10-30 * "amazon-com" ""
  [...]

I would appreciate if you could give me a hint ;)