FoliageInstancedStaticMeshComponent inherits from HISM with hooks for the editor tools and a few other things, most functionality resides there or in it’s parent InstancedStaticMeshComponent.
the params are (async, forceupdate)… async does the rebuild on another thread, forceupdate will rebuild regardless if there is a change or not. I suggested (true, true) to make sure it did rebuild, but in practice you probably dont want to force the re-build.
Honestly, this may be barking up the wrong tree, but it was the first thing that came to mind if the Foliage HISM gets out of sync. You might be right… there may be a bug. I’m assuming the foliage painting/erasing in the editor work fine?
FYI for convenience you can manually rebuild by using the console command:
foliage.RebuildFoliageTrees
You could try digging a spot, run the command, then dig another spot. This would at least ensure cluster tree is updated so you can see whether or not that makes a difference.