Iron Golems swim instead of sink

The bug

Repeated problem with iron golems swimming to top of liquids. Both water and lava.

Steps to reproduce

1. Random golems on my realm server swim next to iron farm
2. Golems swim up in lava on single player world

Note

Not all of them do it, but enough do that it dropped collections rates in farm by 1/3 due to drops burning up because golem was swimming.