Jet,
I've got nothing here, so I am just posting to empathize. I follow the same practices as you and have never seen this happen on any BMW or Audi I have had with a leather wrapped wheel. Bummer.
I am going to take BEM's advice for the leather repair since I have a small area on the drivers seat that looks like someone cleaned it with Poochie's buddies rubbing alcohol...oh well
The bane of buying 2nd hand...
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Current Fleet, 16 F22 M235i xDrive, 15 Audi Q5 3.0 TDI (my Wife's), 07 E85 Z4M Roadster, 01 Toyota Tundra 2wd Long Bed
Retired (a few of them...), 08 E60 550i 6M, 01 Audi S4, 70 Fiat 124, 88 E28 M5, 72 Datsun 510 2Dr, 73 TR6