Slitherine

I either don't understand how it works, or there is some strange features regarding loyalty. To wit: I'm playing Dacia, doing pretty well, in the top 4 nations in both ratings. But, a massive German army attack my norther regions with super strong forest troops. My also strong forest trained Dacian warriors and heavy forest-specialized dudes are on the way, but it takes them three turns to get there. (That's another issue: 3 years to go 200 miles???). Right before they get there, my surrounding regions that were 90+ loyalty, go to 30 something in one turn, and the Germans conquer the province after a 3 turn siege. Now, the recently conquered province (of Balkans) is 100 loyal to the Germans, which means they get their troops repaired. After the attempt to run them off, my troops lose a close battle, and all the regions they could retreat to (not counting the one they attacked from) have dropped to loyalty in the 20's, while the conquered province is still showing 100.
That to me, makes no sense in how people might behave: I can understand freaking out when the massive German army shows up, but what I don't understand is the IMMEDIATE and continuing 100 loyalty to the new conquerors. Seems to me people would be just a little hacked off at having their region wrecked, and a bit optimistic (and rebellious) about having their friends and relatives (Balkans) next door trying to fix things. Oh well, maybe they are actually millennials and don't care about fighting back...
Anyway, it is sort of frustrating, especially not being able to repair the wounds without going 4 regions away. (Which I tried, and the Germans marched from region to region, trashing things and leaving with 100 loyalty!)

some guesses.

I presume this German mob is an independent army not a tribal faction?

if so, an indy army in an adjoining region is a big hit on regional loyalty - so that might explain some of what you are seeing?

Now if the region you lost was German cultured, then I think that might be why they have high loyalty to their new owners - less sure about this.

They are probably independent, this would explain both the fear-induced loyalty hit as Loki says, and the fact that independent and rebels always have full loyalty, it means they don't get rebels themselves (and can replenish easily).

That's right. They almost always seem to be healthy, and never have any loyalty problems...