Well, AI took my land away against all odds.
This is what took place:
1. I've attacked a land of Valandil, won the combat and took it over.
2. Shortly after combat, AI attacked me over the same land, clearly without much chance:
(I was relating that to discord as a funny case of FoT supporting Valandil, hence the screen was taken, I did not yet know what will happen next)
3. Then amazingly, I get this:
And the land is gone:
I assume that somehow the game likely counted in Valandil's army as defending forces, instead of mine.
Especially when you take a look at the bottom of first screen, where the 0 defenders is visible against 375 on the main display.
@Mr. D Is it traceable in your logs?