Just a note, confirming MEB's suspicion, that I've seen the same problem (incomplete or corrupted files) several times in both other graphical software and in non-graphical software, and with a couple of different cloud-sync systems and one corporate NAS-synced system - definitely seems to be a problem with cloud-synchronised or NAS-synchronised folders and moderately large-ish files (and possibly, in at least one case, when rapidly writing numerous smaller data files from the non-graphical software).
Without any definite knowledge, I'm guessing sync cuts in before the entire file is written. Similarly guessing, I am assuming this blocks completion of the local file write (thus the message about unable to add metadata). Writing using append mode, perhaps?
(For real paranoia, I occasionally wonder if [given the remote write is possibly slower than the local write, including upload/transfer time] the remote incomplete file is compared with the local completed file, the incomplete file has a tiny bit newer time stamp and then proceeds to overwrite the local file with the incomplete remote file.)
Not sure this can be handled well by Affinity, except by maybe writing the file first to a temporary file in a different non-synced folder, then copying it to the destination folder, which might introduce a whole new set of problems.
The workaround I've always ended up with is saving to a non-synced folder then manually copying afterwards to the synced folder if required, which I believe has worked in the cases that I know about.