HI MetalD,
As the warning message suggest any instance meshes, like the procedural foliage volume or those painted with the foliage tool support static lighting that is built, but only for the base mesh. Any lower LODs will not support unique lightmaps for them. They will use the lightmap from the base mesh, which depending on the changes for the lower LOD can cause lighting artifacts.
This is simply a warning and can explain some artifacts if you’re using static lighting and instance meshes.
I hope this helps.
Tim