It has been the same case for my s21fe and have found some other user's who has s21 ultra!https://r2.community.samsung.com/t5/Galaxy-S/S21-Ultra-Back-Plate-Coming-off/td-p/12192996https://r2.community.samsung.com/t5/Galaxy-S/S21-back-glass-coming-off...

I also recently upgraded to OneUI 6 and noticed this weired and unsettling change. Is there any update on this?