I have had this happen a few times. I'm hit by a web from a spider, which reduces my dex to 3 for a while therefore lowering my AC. Now, I hit expertise while i'm there to protect myself but when the web wares off and my dex goes up, the AC never recovers, my base AC is now 5 lower than it should be. My AC before entering combat was 20, after leaving it is 15. I have to log off and log back on to restore it to normal.

I'm guessing others have had this happen as i've had it with more than one PC.

It usually happens when 2 or more webs are on top of each other and you fail both.

Just run off the web and run back on, to stick yoruself again, your AC will return to normal.

Or relog, if no web can be found.