Room history is unavailable from tick #11987460 in room E46S42

EDIT: I think I found out that npc invaders can invade through room entrances which aren't connected to available rooms. My towers would occasionally not kill them fast enough because of a missing else in my tower code. I'm still not sure if this was the original problem but I haven't had the disappearance issue again.

All the exits from the room are blocked by a combination of walls and ramparts. At least those exits which are traversable. The times that my creeps all disappear none of those walls or ramparts are destroyed. Recently I saw an invasion happen and my tower killed it easily.

Before:

http://puu.sh/q32X3/d00318d78d.jpg

After the no data:

http://puu.sh/q331v/5a0f0c9c1a.jpg

The western exit is a room currently not available so I don't have it blocked off for now. Also that road disappearing was me contemplating a change in layout, ignore it.

Every once in a while all my creeps disappear at once. This hasn't happened before while I'm looking, but looking in the room history the most recent time it happened everything was normal and then there's a patch of no data. After that patch all the creeps are gone.

I can't tell if there's a bug happening here or what and I couldn't find any thread referring to disappearing creeps without it being a simple respawn question. Any help would be appreciated.