The QR code specification requires an optimal choice of mask to avoid patterns that are difficult to scan. If the rules for this choice are ignored and one of the mask patterns is chosen arbitrarily, but the scanner is not confused by the pattern while scanning it, will there be any decoding problems?
No. The mask that is used is recorded in the QR code itself, in the area around the finder patterns. As long as that is read correctly, the mask will be correctly removed before decoding. It doesn't matter whether it's actually the optimal pattern or not. Any mask may be used in theory.