How to Reproduce:

1. Go to [Options] -> [Accessability Settings] and set the sneak button to toggle.
2. Go back to your world and tap shift.
3. Use freecam.

Effects: The freecam POV will immediately sink down, and it can only be stopped by holding space bar. There is no way to move the POV upwards as the space bar is cancelling the player's downwards movement.

Have you tried tapping shift again? Freecam uses sneak to move downwards, and therefore if sneak is toggled it will toggle going down.