Haha, it’s a bug of course. xD The level select was pretty last second, so I forgot to correctly set the current level variable when entering a level through it after reloading the game or exiting another level. This causes the next level button to point to the same level once every time you use the level select.
It’s fixed in the post-jam build but as of right now the best way to avoid it is to replay the level once and then the next-level button will work again as long as you don’t return to the level select.