pgn has not been designed to only show valid moves, just to allow to show all valid moves (which it does).
So my question to you: how is it serious ? Who cares that you can create invalid pgn files (you can create invalid "any file format" files, if you want to) ?
There is a serious error in the PGN format that allows people to make illegal moves, promote their pawn instantaneously, move pawns backward,
capture nonexsistent pieces, teleport their pieces across the board and even eat their own pieces!! lol
Example: