Ok so I realized early on that the only way to "catch" the AI heroes was with Outposts he'd have to siege so I could come smack him. However, occasionally it seems he just takes my province despite the presence of an outpost with one defender inside. No fight is shown, no summary, just him now owning "my" outpost and me having to siege it back. Anyone else experienced this or know why?

Due to the lack of saves I obviously can't go back and be 110% sure I remembered to have a garrison, however, it's really become instinctive to always buy one unit for a new outpost I build or take in combat, so I highly doubt I've forgotten as often as I've seen this happen...

I don't think that happened to me, only when I forgot to garrison.

did you go bankrupt? I think it automatically dismisses units / scraps stuff if that happens.

Watch out for corruption, its a global variable

It doesn't.

Technically, there are some events/rituals which can damage/destroy fort . Also repairing damage takes time too. But there is no way to lose Outpost with guard without fight.