I expect this is a pretty rare edge case, but mentioning in case it’s an easy fix. I was creating a new post via the Web UI and attached a photo, but accidentally selected an AVIF version instead of JPG when choosing the file. This results in a silent fail when pressing the “Post” button: no message or change on the UI, but nothing posts (and no update showing posting). Under the hood the server is throwing a 500 error but this doesn’t bubble up to anywhere the user can see. Took me a while to figure out because the UI did manage to pull a thumbnail out of the AVIF file so as far as I could see everything looked fine before posting.
Of course this is pure user error but the expected behavior would be an error message rather than a silent fail.