Same here. I only have 500 miles on mine and the first night it was cool enough to roll down the windows I noticed the horrible flapping of the rear center belt. It seems they could have made it disconnect like so many SUVs do. No reason to have it connected 24/7. I did wrap mine around the center armrest and put it back up, but not sure if that will help. I also ordered a padded seat belt wrap to see if that adds weight and changes the aerodynamics of it enough to fix it. I know it is a first-world problem, but something that could have been thought about and designed properly. I am amazed this is the first I am hearing any mention of it.I just noticed the rear seatbelt flapping against the seat with all 4 windows down last night. Pretty annoying.
I saw that here somewhere before, I think they wedged it between the cupholder to fix it, though you can't in an XL.