[Sideway Pawns] Bug: Horizontally pinned pawn can't move horizontally

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In Sideway Pawns mode one can move their pawns sideways as well.
I played white and I wanted to move my pawn towards my king at turn 29. The game didn't let me thinking that the pawn is pinned to the king by the black queen. Is this a bug or part of the rules?
Game link: https://www.chess.com/variants/sideways-pawns/game/48118578/56/1
Position at that move: (not sure if the link above works, nor how to share a variants game)