I had the exact same issue with my 2017 Grabber Blue Mustang. I took it to the dealership and they sent pictures to their Ford rep.OP, did you have any luck?
I am having this issue too...I noticed the window channels last fall - at that point it was only flaking off of the rubber sealant - figured I would deal with the issue in the spring, then Covid hit and threw that all for a loop. Fast forward to today - washing the car at the local self-serve and a large chunk of paint flakes off above the window channel. I'm furious... I'm past the 3 year B2B warranty period, but if Ford thinks I'm going to pay to fix their complete shit workmanship, they are gonna be sorry..... Just look at this!
I had the exact same issue with my 2017 Grabber Blue Mustang. I took it to the dealership and they sent pictures to their Ford rep.
Whole car repainted had 58k miles on it and it was in year 5
Check the third brake light. This has been a known source for water leaks.More bad news found spare tire well full of water. Looks like windows now leaking. GREAT!!!!