No sealant under the Wolfgang Uber Rinseless?
I'm wondering if the Uber is allowing dirt and road film to bond to the paint. I tried using Uber as an in-between-wash window cleaner (since people have praised it as a glass cleaner), but noticed the glass felt like it was getting dirtier between cleanings than when I was using different products or nothing at all. I've since retired it to sporadic washes where I just want to amp the gloss factor a little.
To Eldo's point, I've also heard positive reviews of Poorboy's Bug Squash for a pre spray.
In your situation, if I kept having build up like that showing up in my drying towel, I'd be seriously considering doing a reboot - decon, polish, and probably a different sealant or coating. Just my humble opinion.