The sum of the right front and right rear wheel loadings must...I repeat, MUST...remain equal at all times, both during the launch and while it is parked. Same with left side. If this weren't true, the car would start rolling over. So, assuming the car is symmetrical (left side sum equals right side sum), you definitely have unequal loading of the rear tires with the situation you describe. Yes, you need to back off on the preload.
This is VERY unusual and I'm amazed you were able to accomplish this only with preloading. This usually requires tricks like asymmetric link arrangement and/or adjustment or front end spring rate differences or rebound limiters. Very surprising!
Incidentally, this is no worse than the left front lifting first. It's just that it's very unusual that you were able to more than overcome the driveshaft torque with preloading.