If before the move the destination address contained: 0000000000000000
The source word contained: 1110001110101101
The mask is: 0000000000001111 or in hex 000Fh
Then after the move, the destination will contain: 0000000000001101
Anywhere there is a 1 in the mask will permit the corresponding source bit through to the destination. I’m thinking this might be your issue!