This error has been present for a long time.
Bug:
Sneak mode does not work in flight mode.
For clarity, I took the hatch:

• What should happen:
  You're in flight mode, you're starting to sneaking around. You can put a block on the hatch, without the hatch itself functioning.
• What's going on:
  You're in flight mode, you're starting to sneaking around. You can put a block on the hatch, but instead you have a functioning hatch.