Try going to your options menu and messing with the "Tactical Threshold" slider. It decides what the number of enemies you need to do a tactical battle. If you want it to always ask you if you want a Tactical battle or not set it to 0. Hope that fixes it for you.

Hmmn, I'll doublecheck, but I don't think this is the issue since I've gotten the option to use tactical combat against just one enemy - if the default was set above zero, this wouldn't happen.

Also, to what Klaus said, I've lost these autoresolve battles too.