When using a custom player collision box (by changing it from player.json), the game treats the player as if they have the default collision box. This results in non-smooth and intermittent player movement when there are blocks around. Occasionally, the player's movement stops mid-air, and in some cases, the player glitches into the block above.

If you set the player collision box to be 1 block tall, they will take damage when entering a hole that is also 1 block tall.
Minecraft_2023-07-18_19-45-22.mp4