Zombie Horse With Zombie Spawning

I have a world with 20 block high walls, three blocks thick, walling off a 100,000 block section in minecraft bedrock. The world’s a few years old, though I don’t remember how long exactly. WIthin the walled off area, I covered the entirety of the floor with enough torches to light up everything, and before the update adding spears and zombie horses and zombies riding zombie horses, no hostile mobs, besides some drowned in my non-lit up rivers, could spawn. After the update, I get zombie horses spawning during the night in very well lit up area of my world. I make a copy of the world and entered spectator and look for underground spaces the zombie horse could be spawning in, and maybe clipping up or something? But I couldn’t find anything underground for maybe like 80 blocks in all directions. The only explanation I can thing of is that the zombie horse for some reason disregards artifical light and only follows natural light? But they only spawn in around the same areas too. So far, I’ve noticed two distinct areas where they spawn, and I haven’t seen them spawn any other places. Both places are extrmely well lit, because they’re both areas that my player has to be in a lot of times, one because it’s near my base, and the other because it’s in the road between my base and my farms. Also, there was this one time when a bunch of skeletons and zombies were on the other side of my nether portal waiting for me, and even though no hostile mobs should have been able to enter my portal, they were still there, on the other side of the nether portal (In the same world as the zombie horse thing). That might be relavent, it might not be. But figured I’d mention it.