If you spawn any mob in a closed 1x1 area and put two anvils on them, and ignite tnt on the anvils, the mobs will not die like when you ignite tnt above mobs without anvils.
Linked issues
is duplicated by 5
MC-1882
when creepers explode on anvils TNT around the creeper do not explode
Resolved
MC-4803
If you put an anvil on a piece of TNT and blow up the TNT nothing around the anvil is affected and the anvil is unchanged, You don't even lose health.
Resolved
MC-7022
When lit tnt is in the same block as an anvil it will not cuase damage to blocks
Resolved
MC-19987
TNT does not work properly when inside an anvil.
Resolved
MC-62128
TNT doesn't inflict damage to blocks when an anvil is placed on top of it and lit
Resolved
Intentional. "Minecraft physics."
The anvil has very high blast protection. It's absorbing all of the explosion and thus none will get to the mobs.