Why is one tail pipe so shiny and clean inside

The 2019 Macan S I had did not have the Porsche Sports Exhaust, it had the normal factory exhaust, however I noticed when I swapped it out (briefly) for the AWE Touring exhaust, on passenger side there was an electrical connection to an exhaust flap on the outermost tip; I would imagine this opens up when at wide open throttle or cold start to sound cool, then closes once the cold start is done. If that valve remains closed the vast majority of the time then exhaust gasses will be routed through the muffler and out the other tip. A clean looking exhaust tip, to me, would simply indicate which one of your tips has the valve on it.

Turbocharged cars normally run a bit more rich (that means there's more fuel dumped into the stream than strictly necessary) to help with A/F ratios, that slight excess of fuel will create a sootier exhaust. Also, the turbocharger is lubricated with engine oil and will consume a small amount of oil over time, leading again to a sootier exhaust.

Both of the points above are to say that I don't think having a single exhaust tip that's less sooty than the others is necessarily a bad thing, it would just mean it's the one that stays closed most of the time and gets less "use". Naturally, any concern above that should probably be directed to the dealer to be 100% certain, and for peace of mind.

I happened to come across this link on the forum, and based on it your car is normal from what I see.

From what I can tell, the standard Macan S exhaust has 4 outlets, and only one flapper that opens (inner pipe of passenger side), probably on hard accelerations as others have said, and the inner pipe on the driver side has a permanent restrictor that reduces exhaust out of that port 100% of the time

Starting at the driver side, my suposition on the 4 exhaust outlets are:

#1 Always Open | #2 Always Restricted =========== #3 Normally Closed [Full Throttle Open] | #4 Always Open

You could verify #3 by looking in the pipe after the car is warm and idling and NOT in sport, it should probably be closed. It might be open when you select "Sport" and the car is idling