I was having the same issue…
I was having the same issue with my 2019 GMC Sierra 1500 SLT with 32000 miles dealership told me it needed a triple flush and would cost me $1039.00 they said this was a maintenance problem I told them this was a warranty issue so I carried to my mechanic for second opinion he changed the transmission fluid and filter for half the cost of the dealership and transmission seems to be working find dealership in the business to rip you off on high cost maintenance on issues that doesn't fall in timeframe 1000s miles of GM service guide line