Importing Overleaf to Zotero saw that most citations aren’t found because citation keys are now different from before. Somehow my citation format authorName1998 when syncing with Overleaf becomes author_name_paper_1998.

The solution seems to be pinning citation keys in Better Bibtex: Citation Keys :: Better BibTeX for Zotero

This should guarantee that the keys won’t be automatially (re) generated but stay fixed forever.

This translates to my overleaf export it seems.

Also: when importing from clipboard, the resulting citeKey is also pinned, which is more than logical.