Zombie Reinforcements can spawn in regardless if light level is higher than 9 in nether (hard difficulty)

Closely linked with bug report MC-153787. This however is for zombies spawning even when light level should normally not allow them to spawn, on hard difficulty.

To recreate just go to nether and light up an area so it's above light level 9 and punch enough zombie pigmen. Would recommend downloading donut gold farm for easier testing method.