I have tried multiple workarounds to this issue. I have added retries in the API workflow to add the attachment to the thing with a condition that it will only be added if it’s not already there. I have split adding the attachment to the thing into a separate API workflow and even trying to schedule it to run a few seconds later. But for all cases I see that X number of files are uploaded, resulting in X Attachments being created, but only a subset of these actually being added to the thing.

There must be a bug in the Bubble platform.