In back and forth emails with Tech Support regarding my level 6 barracks that suddenly disappeared from sector 7, it's been admitted that yes, this is a known problem.
If you have bombed the mountains in the NW corner of sector 7 and have built important buildings in those spots, I would advise you to move them right away.
Tech support refuses to supply the gems needed to take my barracks via instant upgrades from level 4 to 5 and level 5 to 6, even though it was at level 6 when it disappeared. So I have to wait 2 weeks while my barracks upgrades to level 6. They provided the upgrade material, but this is far from a satisfactory resolution of the problem
I don't want anyone else here having to go through this. So I repeat. If you have important buildings in sector 7 where mountains were bombed, move them before they disappear and you're stuck waiting weeks to get them leveled back up.