If you are in a 1 block space you can't jump, but if you are on a block without a collision box you can.

Steps to Reproduce:

1. Build a 1-block high tunnel
2. Use water to start swimming and enter the tunnel
3. Try and jump
4. Place a collisionless block (item frame, button, lever, etc.) below you
5. Try and jump

Observed Results:
With a collisionless block below you can jump, but with a collisionless box not below you, you can't jump.

Expected Results:
You shouldn't jump both ways (with or without a collisionless block below you)