AI Settlement Attacks in SFO seem to behave strangely

I'm doing an Empire playthrough on Mortal Empires with SFO (it's really great btw; I'm not entirely sold on SFO, and some campaign starts are straight up snoozefests, but Empire SFO is amazing) and I've noticed some weird behaviour with the AI attacking settlements. The AI will routinely send full army stacks against unwalled minor settlements garrisoned by 10-12 units (and I just mean the settlement garrison proper, not the garrison plus an army sitting in the settlement) and get wiped out. I could understand if it was a full stack of skavenslaves attacking a settlement garrisoned by saurus warriors, but I've seen Karak Kadrin send full stacks after full stacks against the 12-boyz garrison in Gnashrak's Lair and lose. I've inspected the armies they were sending, and aside from 2-3 miners these were decent midgame dwarfen armies led by Ungrim himself which shouldn't have had any problem taking the settlement.

I am aware that SFO makes garrisons stronger and that plenty of buildings outside of walls add units to your garrison, but I've inspected the garrisons of the settlements that kept wiping out full stacks and they were nothing special. At this point I have to ask, has anyone else experienced something similar? Does SFO mess with the siege auto-resolve calculations to give the defender an advantage even above the buffed garrisons?

If so, I'm not sure what to think of it. It would have virtues, like slowing down the steamroll and allowing minor factions to survive in a world of giant blobs, but it also seems to lead to problematic situations, as in my example of Karak Kadrin losing its whole army again and again because it wants to unify its starting province and can't defeat an undefended minor garrison.