You are really correct there. Foliage and lumen alone are both complex systems, and together they can be a nightmare. Unreal’s documentation is okay but it really doesn’t cover the full complexity and edge cases that come up. Part of what I’ve learned is lumen is sensitive to a lot of the nuances of foliage-how the meshes are made, how they’re carded, the materials and masking, the animation, all of it. It’s really easy to get a little mistake in that just FUBARs everything.
Good news: The new engine version appears to fix most of these issues. If you’re having problems currently, you do have a lot of quality levers and options available to you. If you have the time, type help into the content bar and read through the lumen CVars, many of them can offer solutions to some of your content issues, in exchange for a few trade-offs.