This is perhaps more indicative of my own incompetence that it happens to me this frequently, but it has happened consistently enough that I'm guessing it's probably a bug.
At any rate, if you have commentary in the "original title"/"original description" fields when uploading a post but forget to select a rating before slapping that post button, when the page reloads to let you know "hey you forgot to select a rating" the commentary in those fields will be erased.
Nothing else gets erased--the tags, translated commentary, parent ID, and source URL all stay intact, but those two text boxes specifically get thanos snapped.
I don't know if this happens if you're posting something from a web source that has auto-translated commentary, but it definitely happens if you manually have to copypaste the commentary over. It's not the end of the world and it's just a minor inconvenience, but it's probably not the intended behavior of the upload form, so I thought I'd mention it.