Hey, I just caught this bug in Mowzie's Mobs thanks to a report, and I noticed you were doing the same thing I was. I haven't debugged your code but I thought I'd bring it to your attention anyway.

The issue is that AlexsMobs.onBiomeLoadFromJSON() is being event-listened twice in the mod's main class: once when you do MinecraftForge.EVENT_BUS.register(this); on line 63 and again when you do MinecraftForge.EVENT_BUS.addListener(this::onBiomeLoadFromJSON); on line 65.

This might be effectively doubling the spawn rates of your mobs and might explain spawning trouble that a lot of other issue reports discuss. I recommend removing the line MinecraftForge.EVENT_BUS.addListener(this::onBiomeLoadFromJSON); entirely. Hope this helps!