Occasionally, mobs that spawn on, or land on magma blocks, will not take any damage from them. This is a rare occurrence, but can be easily spotted by having a raid spawn on magma blocks.
Mobs that are not taking damage on magma, will not pathfind or ever move. But as soon as they do move(or get moved somehow) then they will start properly taking damage.
Below you can see an image of a raid having spawned on a magma block platform, none of these guys are moving around, and now will take damage/die until they are moved. You can also see a baby drowned, that spawned on a magma block not taking any damage.
This bug hurts mob farms, and kinda breaks the practical uses of magma blocks, since they have one job. To deal damage to things.