if you notice this in the future, you might want to open your browser’s developer tools and look for errors in the console. there were cases in the past were folks had issues uploading image files because of seemingly random connection resets. it was never clear why this sort of thing occurred, but iNat staff effectively worked around the problem by adding code to retry when an image upload failed. i’m not sure whether they added similar retry logic for when actually creating the observation data, and maybe if not, then there could be a chance that random connection resets during that process could result in the sort of thing you’re describing.
this was the thread related to the addition of the retry logic, but this post links back to the other thread with some attempts to troubleshoot the connection resets: https://forum.inaturalist.org/t/retry-uploads-on-the-browser-uploader/34628/15.