I was doing weird stuff in a game; and built some outposts right next to my cities;  as long as they were outside the city walls they were fine; but once they were inside city walls they acquired a buggy behavior: whenever you built an upgrade to the outpost all upgrades at the outpost would disappear.  So if the outpost already had upgrades before going inside the city walls it would keep them; UNTIL you built another upgrade, then both the new upgrade and all prior ones would disappear.