There is a base I made which usually has the Military Outpost marker (Tower with crossed swords). Has a Tower; a Stationhouse; 3 Stormhuts; some labour/craft/foodstuff to be autonomous; + a wall and 10 harpoons of all kind. But sometimes when my group is away it turns into a small X. Not always. Once my group was back in the main city on the other side of the map and it kept its Mil. Outpost marker; then recently after one of the many dives into the ashlands it reverted back to the small X in a rather short time period; and still is there even when the group came back.

I did had something like that experienced before with a fort I made during my campaign against the HN; but that was actually just a Stormhut and a fistful of merc tents with sleeping bags surrounded by a lvl 2 or 3 wall. But here it was continuous marked as outpost until after the war after I removed some of the sleeping bags, base contents and powered it down and moved everybody out. There I explained it to myself that its because I probably removed some critical mass of stuff to warrant its marker.

Another base I which I left unattended for (ingame) weeks now still shows as Mil. Outpost on the map.

... and now while I was writing this and Alt-Tabbed out of the game a few times it reverted back to the Mil. Outpost marker.

no clue
im not sure what causes the different changes.