I recently - last week? - placed a frontier gate frame (without a gate door in it) inside my base and I logged on today to see it had disappeared. When I was placing it the first time, it also disappeared right after I placed it and I had to remake it and place it again.
I tried to place it again today in the same spot that it was and found that I can no longer place it on the edge of a ceiling tile to line up with the wall. It only shows up as able to place in the middle of the ceilings.
I just tried in single player and it worked no problem.