I imagine this happens because the character is under a very low platform, as the character's hitbox is 16x16, if there is a platform on top he cannot jump
tried again, but that doesn't seem to be the main factor.
However, it seems that the collision system can't differentiate between vertical and horizontal collisions (meaning you can't jump from a corner because the raised step or wall that forms the vertical part is treated as a too-close-to-the-character ceiling)