The upper half of a player's hitbox can occasionally pass through 1 block high spaces, without suffocating. This allows for an 'x-ray' ability and the ability to jump into ceilings. This does not apply to the lower half of the hitbox and also occurs in creative mode when either flying up or walking. This does not seem to affect mobs. There have been similar issues rarely occurring when a player can pass through a wall, and does not seem to be affected by lag.

Thank you for reading this bug report and I hope it can be fixed soon.

Edit: this requires no special circumstances such as elytra flying. It occurs randomly and seemingly for no reason