I also want to add that the pins were both in the piston.I think your missing what I'm saying for break in. mix your fuel at 50/1. Like your running without injection. Plus you have the injection. This makes a very rich mix to help the beak in. Effectively double oiling for the first tank.
Usually a ring catching a port is due to a locating pin falling out and the ring spins and the end of the ring gets into the port. If the gap was very tight on assembly it could also cause it to possibly catch in the port as it heats and expands.
.012" is about what I would expect for gap. So that rules ring gap out.
Now the added info on motor having burnt this cyl. Being reason for rebuild. That starts to shed light on the issue. That leads me back towards an air leak on that cyl. Leaning it out. Maybe intake manifold. Reed cage assembly. Case half seal. Cyl base gasket. Only way to know for sure is to pressure test motor.
This also leaves possible issue still with carb.
I don’t have any schematics of yours or a service manual. I do mainly tigersharks but i have to ask? Are any cylinders new? If so, are the ports chambfered to the proper degree? I know that will almost guarantee ring catching mainly on exhaust side plus a out of spec. ring endgap adds to disaster.