Source: http://steamcommunity.com/app/289130/discussions/0/208684375420567771/

Anyone else experienced this bug ? Would be some kind of game-breaking and should be fixed very soon...

Yes it seems to be systematic, after every siege the enemy units are at least at half health and they're at around 10% if the siege lasted two turns. I've seen some entire garrisons die before I even attacked (because fortification value was at something like 600)... it's honestly quite gamebreaking.

Experiencing the same.

And as a guy who likes to turtle with fortification and retaliation this is a real buzz kill.

Also it makes many buildings/perks/items, even faction specific ones irrelevant.

This is a real game breaking bug.

Had a city with 3000+ fortification, AI sieges, My garrison is wiped out in 3 turns...  Enemy just walk inside the city the next turn...

Wish they got HOTfix this one, as it is really ruining the game.

Thanks for your report, we'll look into this ASAP.