MythicMobs interferes with villagers spawned by 3rd party plugins.
Running Minecraft 1.16.5, I've tried both the MythicMobs versions which still use MMOLib and the newest version which uses MythicLib. The result is the same.
I'm using a plugin called RainbowsPro. The idea is that a rainbow will spawn at a random location in the world, and at the end of that rainbow is a Leprechaun. The Leprechaun is a Baby Villager.
With MythicMobs removed, this plugin works just fine with no issues. Nothing interferes and it works like a charm. However, with MythicMobs installed, the Baby Villager will only spawn (or perhaps not instantly despawn, hard to say which is happening specifically) if it is spawned near a player. If it's spawned, say, 500 blocks or so from a player on an unloaded chunk (the world is pre-generated, so I don't mean ungenerated), it won't show up at all. The Baby Villager will be completely absent.
I've done various things to narrow this down to being an issue with MythicMobs and the result remains - MythicMobs installed = Issues with Baby Villager spawns. MythicMobs uninstalled = No issues with Baby Villager spawns whatsoever.