Zombie Pigmen don't attack if you place fire next to or on the block they are standing on

What Happened:
I used flint and steel, lava, or a fire charge to ignite the block a zombie pigmen stood on (naturally or unnaturally spawn mob). They didn't recognize the player as being the source of the fire, and will not attack.

What I Expected to Happen:
Zombie Pigmen attack if you attack them with fire even though the fire does not directly damage to them.