It's the \ldots#m entry in tex.cwl that "pollutes" the one without #m in latex-document.cwl. Both tex.cwl and latex-document.cwl are loaded by default.

$ rg --no-heading '\\ldots\b' completion
completion/latex-document.cwl:254:\ldots
completion/plaintex.cwl:388:\ldots#m
completion/tex.cwl:572:\ldots#m

This can be resolved by removing the \ldots#m entry from tex.cwl, as it's a plainTeX macro and is already recorded in plaintex.cwl.

We may need to review all the entries which are both recorded in tex.cwl and latex-document.cwl, but with slight differences.