So we have our War in HIP 20638. Except, you guessed it, no combat zones or combat missions.
Sigh… Filing a bug report after the next tick if nothing changes but I don’t like our chances, a little google and this seems to have been a known issue for many months that occasionally occurs and hasn’t been fixed.
It might be worth doing regular missions and killing system authority to see if that has any effect, although according to the rules it shouldn’t.
Update: Apparently this issue has been fixed in 2.1. We may have to wait until then to take the system. https://forums.frontier.co.uk/showthread.php?t=242678