well, it would have been nice if it worked 
see Unity Won’t Fix
Unity Issue Tracker - Terrain detail meshes do not receive the correct lighting when using Light Layers in 2021.2 (unity3d.com)
Well the issue is marked as not being fixed in 2021 cycle
For 2022 forward it’s fixed. You can wait on that or use the current beta leading up to the release if you can afford to do that.
I’ve never tried to use terrain detail meshes with light layers so this is the first I’m even finding out this was an issue.