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In fact, this is SO BROKEN because it does not happen to PC characters! I took Seagard as the Lord of the Crossing. I built ONE province improvement. My idiot king gets in a war he can't win, and naturally Seagard and the Twins are sacked. I lose 3 province improvements in Seagard, when I only built one.

Who is responsible for this??
 
I never realised it, but if it's true, that's not very fun. I suggest replacing it by a Sacked modifier and sometimes, an event that destroy a province improvement.
 
It's true unfortunately. I played as the count of Wreckstone in the Stormlands (little island off Cape Wrath) last week, and spent 150 game years building improvements. I FINALLY was where I could expand... then, the Reach invades the Stormlands, sacks my county, and sets me back to no improvements. I decided to start over and it happens today! It's seriously no fun.