I came across this issue while attempting to play on a 1.14 SMP server. The hopper hitbox can trap you in a "crouch mode" (underwater) in which you cannot escape while in survival. The player seems to be forced in crouch mode without control to free from this mode.

https://youtu.be/1t8ZySsOITw

Steps to recreate:
Go into a water 2 or more deep. Place a chest (or stone wall), place blocks on 3 sides, place a hopper above the chest. While swimming, swim up to the chest and stop. As soon as you go into crouch mode you are stuck and cannot move away.