Steps:
1. Enter a zone where flying is possible.
2. Fly up and stop moving while in the air.
3. With controller, change to cross hotbar set 1 (even if already on cross hotbar set 1).
Character will move up slightly. Since change to cross hotbar 1 is (R1 + △, PS4 controls), and △ is the move up button while in the air.
Situations where the bug does *not* appear:
Neither (L2 + △) nor (R2 + △) moves the character in the air.
X is the move down button while in the air, but change to cross hotbar 3 (R1 + X) doesn't move character down.
△ is the jump button while on the ground, but (R1 + △) doesn't cause character to jump.
So the bug only appears with the unique combination of:
1. in the air
2. (R1 + △)