Okay, yeah. This is a separate bug, and I'm honestly not sure what causes it. I've been working on fixing it in other instances lately, however nothing I've done to mitigate it seems directly linked to anything that could actually cause it. I've also noticed that if you try to force ground battles past this bug, it always initiates as a space battle on a planet with the correct ground forces, but in space transports. Unfortunately I can't say how to fix it at the moment, but the more information you (or anyone else) can provide when it occurs for you in 2.1 would be very helpful.