I don't know if it's just on 20w21a or all version since f3+n,  but if you turn gamemode into spectator using f3+n or f3+f4, and then do f3+n or f3+f4 again, it moves you. This doesn't happen with /gamemode spectator then /gamemode survival. 

To re-create:

1. Crouch on the edge of a block, and then hold either w+a; w+d; s+a; or s+d in order to go right into the corner. 
2. Let go of the w, a, s, or d keys followed by the shift key.
3. F3+N or F3+F4
4. F3+N or F3+F4 again
5. Watch as you fall off the block