Unsure whether this is a bug however when I lightmap my level GG is insisting on mapping vast areas of terrain where there's nothing going on. No entities, no terrain features - just flat and featureless. It's miles away from the player and frankly I'm not bothered whether it's calculated or not. Of course, I'd rathr it wasn't as it means it takes a long time to map a fairly simple level.

Does this really need to be lightmapped?

I'd suggest this could be resolved by allowing the user to define the area to be calculated - like in Blender where you can choose to render within a box selection, for example.

does this still happen if you load up GG and then load the map from fresh?
i think it's likely just models from a different map or something that confuse the grid as most of the time the light mapper grid works fine for me.

This is usually caused by a scale factor other then 100 in an FPE file.
One of the very tall background buildings from one of the Mega packs or the building pack.