How have you set up your Google Sheets writer?

I don't think it's any different from all the other writers I have on the workbench. I made sure that the fme_db_operation is 'UPSERT' and not 'INSERT'. See attached picture below:

Hmmm... that's how I have them set up too.

Can you post a screenshot of the entire workspace? Are you by any chance reading and writing to the same sheet?

Here's the main part of the workbench that's a mystery.

Yes, I'm reading and writing to the same google sheet. I'm also using a change detector in the middle

Since you're doing an upsert, are you sure that row attribute is present when you're writing?

For the row attribute, I typically leave it at 'google_sheets_row_number' or blank. The latter I thought automatically adds a record after the last row with data.

Yes, that's what it should do. I think best option for now is to inspect features before writing, see what it says for the row number and then check whether or not they're actually written.

If you could bring it down to a small example that would really help in trying to find the cause for this.