Since the destination contains the same bit set as the source, I'm guessing that the MVM could have been changed from a MOV, and that no-one cleared the bit that was inadvertently MOV'd.
MVM is most useful when you need to merge bits into a word of bits that could be used for other purposes. Predominantly used to set a specific pattern of bits into an Output data word (eg. for actuators or relays), without overwriting the contents of the other bits, that could be used for other devices.
If you genuinely want ONLY the bits specified by the Mask value copied to the destination, then consider using the AND instruction, with Source A as your data, and Source B as your mask. This would automatically clear any bits in the destination that you aren't interested in.
Without knowing more about your data-handling, and what you are expecting, it is hard to advise conclusively.