cooking times

Multiple racks shouldn't affect cook time appreciably. My loin backs take about 4 hrs at 250*. At about 3.25-3.5 hrs, check the ribs for doneness. Run a toothpick through the meaty part of the ribs. If it slides in and out with no resistance, they're ready. Otherwise, smoke them for 30 more minutes and recheck.

Mine take about 4 hrs from a cold start, so yours being done before 4 hrs with a preheated smoker isn't surprising. Just check them earlier to make sure you're taking them out of the smoker when they're ready. Fickled things those pigs.

I'm just not a fan of trying to use times to determine doneness. It's just never reliable.

Shape and thickness matter. Was there enough size difference that it was thin and just overcooked?

The toothpick test will help.

And I always suggest while you learn and until you're a rib expert, to weigh the rack, keep good notes. Test them before they are done as you can always add time.