Hello, I have the same issue with the citations unlinking when working in Google Docs. It is definitely not just caused by the lack of Zotero Connector. I just now finished re-linking the citations manually, uploaded the document on Google Drive, edited it from Google Docs (just added a letter) and the citations got unlinked, even though I have the Zotero Connector on my browser.
What do you mean by "uploaded the document on Google Drive"? If you're saying you took a Word document with Zotero citations and just uploaded that to Google Drive, that will certainly break all your citations. Moving Documents with Zotero Citations Between Word Processors explains how to transfer documents between word processors.
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Could you please investigate this?
https://www.zotero.org/support/kb/debugging_broken_documents
The citation unlinking happens not during the upload, but when Google Docs saves the docx file (i.e. after a small edit).
That's the only way to transfer to Google with citations.