I wasn’t able to see this initially but when looking at SM_Boulder05a I was able to see what was going on fairly easily.
Each LOD needs to have the lightmap generated for it. You just need to go enable Generate Lightmap UV and hit apply for each LOD. That way the LODs can have the lightmap set for them as well. Without this, the LOD wouldn’t have a lightmap other than being able to use UV 0 since that’s the only one available to them all.
This isn’t a bug though, just maybe not as intuitive a process to know why your UV wasn’t being set.