Verification builds: 1.5.3 Xbox one / Nintendo Switch, 1.6.0.30 Win 10.

Summary:
On controller and creative mode the player can get into a sneaking / crouching state while flying, where pressing the toggle crouch button (Fly Down Slow or default RS) will not toggle the sneak state into not sneaking and the player will be forced downward continuously.

Steps to reproduce:
Creative, use a controller (on Win 10 a Xbox one controller is used)
1. Make sure the player is walking on a surface (not flying)
2. Click RS Right stick (default or reassigned Fly Down Slow)
3. The player is now sneaking / crouching
4. Double tap A (default or reassigned Jump/ Fly Up button)
5. The player is now flying while sneaking / crouching
6. Notice that trying to change the sneak state with RS (default) is not working to undo the sneaking position.

Expected behavior:
The sneaking toggle to be off when entering fly mode or RS (Fly Down Slow) to function to toggle the sneak mode.

Screenshot / Video's attached:
Minecraft 19-8-2018 17_49_04 clip.mp4

Additional information:
The player can get out of this state by pressing B (default or Sneak / Fly down) or by double tapping A (Jump / Fly Up) again and pressing RS (Fly Down Slow).
Some players might assign a different function to the B button (Sneak / Fly down) as a similar action can be achieved with RS (Fly Down Slow).