I've never soaped any of my bikes. Ever! Never gets that dirty in thr first place so no need. Just a wet microfiber towel to wet the fairings without having to hose it down and a couple of dry microfiber towels to wipe it dry so that can't be it either.
This in my opinion is like one of the many odd quirks of ducati ownership. People complain about spongy clutch and I've never experienced that problem. Burnt lower right fairing? I've swapped mine out twice and some have never done so, even without the spacer fix. Some complain about hot start issues and claim removing the canister is a fix and i haven't experienced this since my first service but obviously others have and some even continue to have the problem even after removing the canister. This rear hugger .... is just another quirk without a 100% known fix.