At 2500-3500 RPM the car will rev freely, but then "catch up" and engine speed will match drivetrain again. Car launches from a stop just fine, which is why I didn't think it was a worn clutch. Any ideas?

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2013 Audi S5 Quattro 3.0L Supercharged V6 with a 6 speed manual. At 2500-3500 RPM the car will rev freely, but then "catch up" and engine speed will match drivetrain again. Car launches from a stop just fine, which is why I didn't think it was a worn clutch. Any ideas? So here is how this goes down: Stopped, 1st gear normal launch, granny shifting into second gear, second gear the car catches and moves normally, then at about 3000 RPM, the engine just revs up, meaning the engine and the tires no longer feel connected, then on it's own without doing anything other than continuing to move forward, the engine slows down and "catches" again. Things I've considered: 1. Bad clutch, but it launches without any issue so maybe not? 2. Bad axle/joint/hub on one of the wheels? It's AWD so maybe the engine is sending power to a busted axle, then realizes its not gaining any traction and the computer sends power somewhere else. This is my first AWD car though so I wouldn't know if this is it.