its not a bug
Bug with freeze cancelling out checks

that happened to me too. freezing a piece and taking a piece that is protected by it is illegal. it is also not a good idea, considering that all they have to do is counterfreeze you and that's checkmate
it should be legal, since freezing counters checks, but you are right that it would be a bad move
https://www.chess.com/variants/spell-chess/game/53970879
on move 13, i could freeze g5 and take the queen, but it wouldnt let me do that