The bug
When standing inside a filled cauldron, endermen, blazes and snowmen don't take damage.
Code analysis
Based off of Minecraft 1.11, MCP 9.35
The method net.minecraft.block.BlockCauldron.onEntityCollidedWithBlock(World worldIn, BlockPos pos, IBlockState state, Entity entityIn)
would need to be changed.
It would be good if a method was added for entities to indicate whether or not water harms them. This way all the entity type checking could be prevented.
Confirmed. Clones MC-3871.