You can bump your head against a four-high block

Cannot reproduce. Can you post a screenshot of your setup?

I mean that when you run in a four block high hallway, and you jump when the ceiling gets 3 blocks high, you should be able to keep sprinting. With this bug, you'll bump the top of your head against the part when the ceiling goes one block down, and you won't sprint anymore. Or do you jump higher when you're sprinting?

you do jump higher than one block, that's the simple reason. for example you can jump on a trapdoor which is placed one 1 higher than you.
Actually the only bug I see here is that if you sprintjump you are faster than by only jumping, where you should actually be faster.
Especially in 2m high ceilings this leads to the ridicolous behaviour that by spamming the jumpkey and constantly bumping your head against the ceilig while sprinting you are alot faster.

I just figured out you also bump your head if you aren't sprinting

Indeed, you jump 1,249 blocks high, and if you're in a three block high ceiling you can only jump 1,200 blocks high, so this isn't a bug