I replaced the screen protector on my S21U yesterday and I had to remove the phone from its protective case to do so. The back of mine still looks brand new, same as it did over 3 years ago, no back separation or anything.
Also, here's something to consider if you ever leave your phone inside your car on a sunny day with the windows up. Temps like this might cause the back to separate.
Even on a beautiful 75-degree day, the temperature after 90 minutes will be over 120 degrees. If you're in a desert heat wave and the outside temperature is 110, expect a car interior to be around 160 degrees. Since these are averages, there's about a 50-50 chance that your car could get even hotter.