0.983 AI builds outpost in my territory

The AI build an outpost in my territory, right next to my capital. Not that it should bother me much because it was immediately converted back next turn, but if I had higher level improvments on my resources I had to build them again. Screenshots and savegames before/after are available. 

Possible hint: My faction, not the enemy, has the arcane outpost (or what it is called) ability and I used it multiple times.