OP this is late but I just saw this, sorry. I've had the same thing happen. Fortunately I saw the J wrap pulling out and about the first 5' or so pulling back in the wind (driver side). I caught it as it started because I was wary of the sides (lost both fenders - first one on its initial voyage). The J wrap didn't come all the way out because it was firmly held by the vertical trim piece going up the back corner.
I pulled the transition strip and all the screws just went into nothing and the J wrap was barely sandwiched under the strip at the bottom. Loosened the bottom supports, had DW push the strip up as high as it would go and put rivets in it - it did NOT move after that. Reinstalled the transition strip, fabricated new supports for the bottom then sealed the top of the transition strip. Never flexed again in the following 5 years I owned it.
Danny and Susan wife of 55 years
2019 Ram Laramie 3500 6.4 4x4 CC 4.10 SRW
2020 Montana High Country 331RL