Why don't you start a new file, create a text box, enter a sentence, then whack Return (creating the overset issue) then save the resulting file and post a link to it here? We can do questions and answers to figure out your issue, but you say you've already tried everything.
My hunch is No Break in local formatting, or maybe a character style. (Or maybe in a Next Style?) You didn't mention No Break, so it's just a guess.
There have been some notebooks out there, and this is probably one of them, that map both the alpha Enter and the numpad Enter together. Pressing the numlock key should change the behavior if that's the case.
If this is what is happening, you can edit the keyboard shortcuts to change both keys to insert a paragraph break (numpad enter is a column break by default) and create a new shortcut for the column break if you need one.
No Break isn't having any effect. And I've checked styles. Everything's at the default values.
I'll have to see about this. My laptop doesn't have a numpad, so it doesn't have numlock either.
I've already had to make custom keybinds to give my enter key extra functions (such as using the numpad enter behavior when Enter+function is pressed). I'm going to check if something got screwed up in the custom binds, and also try creating a numlock bind see if that helps.
The custom fn+Enter bind I created to make the normal enter/return key have the numpad enter column break behavior when I needed it just turned into the normal function for the enter/return key. It decided to change itself and get rid of the condition that required the combination of the function key and enter.
I don't know why this happened, and it will probably remained a mystery.
Anyway, it's no wonder I couldn't find what was going wrong with InDesign, because it wasn't InDesign that was having the problem. It was a issue with my system's keybind registry files.