2015 Nissan Sentra High RPM Issue.

My mom recently purchased a used 2015 Sentra automatic with 13K miles. She claims that on a few occasions when accelerating from a stop that the rpms will rev high in the 3k to 4k range and it will take off sluggishly until she lets off the gas pedal and tries again.
She has an appointment with dealership next week but is getting on my nerves freaking out over it. She swears the transmission could be defective. It has done this 2 times.
She has been nitpicking the car from day one. Yesterday it was something about an odor when turning on the ac. A week ago she was complaining about a rear defroster line being defective. The car has a warranty. I'm not even sure if anything is wrong but would like to ask for any opinions on the issue.:runaround:

Odor when turning on the AC could be a dirty/moldy cabin filter. The trans issue might just be her not being used to a CVT trans.