Platformer Characters are coded so they can work on all framerates, however when going too fast, and going under something. They end up getting stuck.
Now, this usually doesn’t have to be an issue, but players on lower framerates have this problem when going normal speed whilst sliding.
I’ve ensured the hitboxes are correct AND I just started making this character, meaning it’s not something I’ve done.
TL;DR: If you’re character is going too fast for the current framerate, going under tight gaps is impossible even if the hitbox fits.
https://files.catbox.moe/sr6d90.mp4 (video of the bug in action.)