Somehow I got above 3841 levels when playing a modpack with OpenBlocks (it was Antimatter Chemistry by defeating the dragon around a hundred times if that helps), and when I tried to drain them with the XP drain it got stuck at 3841 levels with a full bar, even though the tank beneath was still being filled. It is probably not related to the other mods in the modpack since I managed to reproduce this issue when playing with OpenBlocks only.

Also I don't mind if you don't fix it since that basically makes an infinite XP well but I wanted to let you know regardless