Custom modpack with Eidolon (0.2.7) and Traverse Reforged (0.4.3.8) on 1.16.5 (forge 36.1.18), single player.
After setting up shop and going through 3 nights in a Traverse Reforged biome (AutumnalWoodedHills, but confirmed in a few other, flatter biomes with fewer trees), and upping the spawn rate considerably in the config, neither a brute nor a wraith had spawned. Upon finding and camping out in a vanilla Plains biome, spawns of these two were frequent.
Not sure why this would be the case, considering that Eidolon seems to sub to the BiomeLoadingEvent in order to add these, and the forge docs indicate that this should work. And I've confirmed in the Traverse source that those biomes are registered as type Overworld.