Was able to reproduce on 1.20.41, the default locations for d-pad is in an area that can't be saved in, so slight edits to the defaults are impossible. The red zones are far too big making the available options to move it into extremely uncomfortable for actual use.
I think I know why the controls reset. It’s because of this bug.
I assume at any point on multiplayer, the controls moves in restricted areas and they reset rather than staying in that position. For 4:3 screens, blame mojang for making the d-pads bounding box too large. But if it was fixed, then changes made to the controls will be saved at all times rather than resetting on PvP situation.