The bug
For context I am fairly familiar on a technical level with the spawning and behaviour of Zombie Reinforcements. I have a Gold Farm in a Nether Perimiter and have found the odd Zombie here and there in areas that should be spawn proofed (either glass or light levels above 0).

After LOTS of testing with a second account to spec while I eventually summoned some, I isolated the blocks they are spawning on and was able to narrow it down to the light level being 7 or below (instead of the expected 0).

This was tested in the Nether in both creative and survival mode with both Zombified Piglins and Zombies.

• Confirmed to be present in 1.20.1 & 1.19.2

To reproduce

1. Create a new world in Hard Mode
2. Go to the Nether and clear a space
3. Create a block wrapped with glass and summon a Zombie there using:

   summon minecraft:zombie ~ ~ ~ {Attributes:[{Name:zombie.spawn_reinforcements,Base:100}]}

4. Create a line of spawnable blocks away from the Zombie with a row of glass panes on top and glass on either side
5. Add a row of light sources (glowstone) on the 4th block above the glass panes (3 blocks of air in between)
6. Throw snowballs at the caged Zombie
   → Zombie reinforcements spawn

Recap

Looks like the Zombie reinforcements lightl level requirement update in the overworld (7 and below > 0) wasnt replicated in other dimensions?

I would have thought it would be 0 in all dimensions seeing as its regular Zombies spawning (who dont spawn naturally in the Nether and therefore dont have an obvious Nether specific spawning rule applied). 

This can cause major grief in builds due to the spawning position of reinforcement Zombies (north west corner) since a block that has a pane or wall above it (blocking regular spawns) allows reinforcements to spawn if the light level isn't above 7.