I think what is going on is that the progression between zombie to villager is not completely done right. where as by they act look and sound like an npc but still have some command code from the time they where an zombie which still lingers. which would explain why they disappear and dont reproduce because that is what the zombie villager would do. thankfully my npc has not diaper (yet), has been graded by walls and golem and on normal survival and yet it still wont reproduce. even with doors a plenty.
It is also must mention that this has never happen to me during the last time i played minecraft which was 6 months ago (give or take). only in this patch has this ever happen
This has clearly has not been resolved but it seems skins are more important then npc
_