Tags stop working for specific file (path disappears from tags option) after manually setting tags once in that file

[fermifunktion]

I found no help on this specific problem elsewhere, so I decided to ask my question here (sorry, if this is the wrong place to ask):

after setting tags manually in a specific file (set tags+=;$HOME) and leaving vim, tagsonly is showing
tags=./tags,./TAGS,tags,TAGS after reopening the file, but not the project specific path gutentags added before.

How do I get gutentags to work on that specific file again? Does vim remember specific settings to individual files somewhere?

Many thanks in advance!