A different, but I'm guessing related, problem showed up for me working with a document that I've been working on for months that has ~130 references in it. This is a bit lengthy, but I've already started some troubleshooting to figure out how bad the problem is.

In-text (not footnote) references starting at about # 100 don't show up as fields in the document. If I click within them and hit add/edit, it starts to insert a new one rather than edit the existing one. The first ~100 references in the document are fine. The field appears to actually be there, though. I'm using a numbered reference style now. If I switch to a "Smith et al" type style, the new citation text shows up, but is also just text and not a field. The numbers from the original style stay there (so, e.g., it would say "100 Smith et al"). If instead I insert a new reference earlier in the document (so all the reference numbers should increase by one), a space appears after the existing numbers (so rather than "100." it would be "100 ."). If I select that new space and hit add/edit citation, it will let me edit the corresponding citation. These edits do not, however, impact the actual text in the document.

I'm using Word for Mac 16.9. Updating to Zotero 5.0.34.3 did not help.

@mrakins: I've moved this to a separate thread, so you can continue here if you're still having trouble.