So I was wondering: is he getting it wrong, or is there really an alternate pronunciation m̄-mián which is unrecorded in either dictionary?
This might be a case of assimilation, where the b nasalizes because of the m in front. Technically speaking, the syllable mián is "not allowed" because in most varieties of Hokkien, [b] and [m] are allophones of /b/, where [m] only shows up with a nasalized final, e.g., 滿 /buã/ → [muã ~ mua].
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.