So I was wondering: is he getting it wrong, or is there really an alternate pronunciation m̄-mián which is unrecorded in either dictionary?
When pronouncing, both of them are okay since we can tell what (s)he says. But it is informal and incorrect to say mián. We know that /p/ in hokkien is same as b in chinese pinyin, and /b/ in hokkien sounds near /m/. Therefore, /b/ sometimes sounds like /m/. But it is wrong to pronounce or write /b/ as /m/. Pā-bián sounds like mián.