I'm testing a lot with different mobs in order to make some fancy addons. Every time I want to kill all minecraft:villager_v2 mobs, every minecraft:villager will die. I don't know why the game is that confused about the difference, but I guess it's because you wanted the villager_v2s to listen to "villager" e.g. in /summon, which is totally nonsence at my oppinion btw, because every old map now spawns the wrong mob.

However, I hope this will get fixed one day, because working with the minecraft:villager mob is a mess right now. Just make it 2 mobs. Whoever can type "/summon minecraft:villager" can also add a "_v2".

Regards

Vinzent