I have a 2016 Yukon Denali with 50,000 miles, I have an extended warranty and the drive train is still under manufactures warranty. The truck went into the dealer for hard shifting from start. The dealer ship had the vehicle for 2 weeks. Now I have smooth shifting from start, but downshifting between 5mph and 3mph coming to a stop there is a hard shift. The dealer is telling me because it is a learning transmission I need to wait 400-500 miles. Is this true?