Block the airplane up on a level floor and do some measuring. Check the incidences.

Also, in both turn directions, the right wing is the one stalling and "falling". And in turns, the AOA is increased and it's AOA that causes stalls. If your right wing is set with more incidence than the left, it'll naturally stall sooner. In a turn or in a straight line. You'll more often see it in the turn, because we do that twice every lap, right.

When you slow the model down as slow as it'll go until it stalls, which way does it stall? I bet it's to the right almost always.

A lateral balance check is also a good idea..... and you've done that. With a low wing, however, the cg can have a bit more effect in the air than we might see in our lateral balance check.

Have you measured the aileron deflection of each side? One moving more than the other will act as described. And if you've got a weak servo on one side, it'll do that. Or a servo with some teeth missing. If you don't have the same servos side-to-side, one side will be weaker.

Also, aileron gap will do that. Seal the gaps on both sides. Do that no matter what the measuring and checking show. It'll take no more than a half hour and you just might be amazed at the results. I use clear Ultracote and it's dead simple to do. I iron it into the gap and then just trim it. Don't need to iron it out onto the wing or aileron. And then run the iron along the seam and they smooth out perfectly. Ultracote in the gap is quiet an strong. No wrinkling sounds or anything. It's like it wasn't there.... until you fly.