MC 1.12.2
ERZ 1.3.5
My test subjects were the sprite, greater Sprite, and frozen knight. Even after setting allBiomes to false, adding aether_legacy:aether_highlands to their whitelist, and jacking up spawn weights to 100 for each mob, nothing ever seemed to spawn anywhere in the Aether.

By the way I think the frozen knight should make skeleton noises instead of zombie noises.

Ill check into it.

Note to self https://minecraft.curseforge.com/projects/aether-legacy/files/2520541

Good call on sound too

i know this was forever ago. but ive tested this off and on a few times, and yeah i they dont spawn.

But.. This just works fine in other dimensions. nether,end, mods like hunting dimension. Dimensions such as the ones in the Sevtech Ages modpack.

so I think Aether legacy must be doing something specific/special to control the dimension, it has to be fixed on their end. Sorry.