Steps to reproduce

• Enter swimming animation while in 2-block deep water or greater.
• Die while swimming (in all my cases I've drowned)
• Respawn w/o exiting to menu.
• Attempt to collide with blocks above you (i.e. jump into a block from below, run into a block that's 1-block above the ground from the side), and find you're able to clip through it.

Temporary solutions

You can either exit and reload the world, or enter a body of water and start then end the swimming animation to fix the hitbox.

I am unsure if this affects multiplayer since this has only happened to me in single-player thus far.