thanks for anserwing Helen,
none of these 3 conditions were met, so sync should have archived. 'max_file_size_for_versioning' is set to 1000. (I'm guessing it's MB, in which case no file could exceed that)
archive is enabled and it did save the project, but an old version from the day before. which makes it useless.
On Kuba's computer it saved some unrelated things.
Before we noticed this, this happened: we were working on the project, we saved, then closed the project, opened a different project (not from a sync folder) and then went back to the previous one to see that all the files we added today are gone.