I know this is an older thread, but my new-to-me 2020 Ram 2500 was also exhibiting the rear corners of the painted Rambox sides coming off/separating, and researching the issue is what brought me to this forum. There is an updated TSB (No. 23-051-22, dated November 30, 2022) where the painted applique and the box lid itself are both replaced if the issue is noticed, attached below. This TSB supersedes TSB 23-019-19 Rev. A and RSU 19-044, and is applicable to all 2019-2022 Ram 1500, 2500, and 3500 trucks built on or before March 30, 2021.
I noticed on my boxes that the applique sections appeared to be inline with the rest of the bed, but the Rambox lids themselves were actually warped upward, causing the separation. I believe that's why Ram now replaces both the lid and the painted applique. My truck is still under warranty and the dealer is replacing both box lids, so I hope the new lids are reinforced/redesigned to eliminate the issue. Hope this helps others frustrated by this flaw.