Resolution: Cannot Reproduce
What I expected to happen was...:
Villager would sink and stay on magma block at bottom of water
What actually happened was...:
Villagers shot up to the surface of water as soon as they were spawned, and aren't sinking with magma bubble columns under them.
Steps to Reproduce:
1. Place magma block under water source blocks to create a bubble column
2. Spawn Villager on magma block (using command, spawn egg, etc.)