Transmission Shudder problems of the 2016 Chevrolet Corvette

I have a '16 Corvette z-51 w/8-speed auto trans. Ordered car brand new in April 2015 - took possession in sept. 2015. Car turned 6,000 miles in April 2021. On a drive recently, rear end began to experience a shudder/ vibration between 50-65 mph like driving over small rumble strips. Internet research resulted in me finding the existence of service bulletin no. 18-na-355, dated February 2019 and an updated bulletin with same number dated April, 2020. Problem is a known problem for a host of gm vehicles from 2015-19 with the gm 8l45 and 8l90 8-speed automatic transmissions. Apparently the wrong fluid was installed in these transmissions. Reference attached bulletin 18-na-355. Took car to dealer the following Monday and it was diagnosed as having the stated transmission shudder problem as outlined in service bulletin. Because my car is out of factory warranty by 6-months and gm extended warranty won't cover for this expense, I was left to pay the $650 to correct a known issue. Why must owners pay for gm's error? attached bulletin explains the steps taken to correct the problem, involving 3 flushes and installing new/ different type of transmission fluid - new part number 19417577. Time will tell if this so-called fix works. Internet auto forums indicate some folks have had to return to the gm dealer more than once for this problem, after the flush procedure. Contacted gm and logged formal complaint -- with no response and basically nothing but a run around thus far. As a consumer who paid good money for a high end performance vehicle would it not have been good business practice to notify customers of a pending problem rather than remain silent about the problem, let vehicle warranty run out, then tell customers it's their problem? poor customer satisfaction policy no doubt! gm should be forced to own up to this problem and pay for the repair.