The case of my Samsung Flip Phone cracked after being dropped on a hard floor, resulting in an intermittent fault. So I purchased a third party case to protect the replacement phone. However, I noticed that when folded to 90 degrees (halfway closed) the hinge mechanism appeared to be putting pressure on the phone's hinge.
After a few days the screen protector started to lift and bubble, along the fold in the screen. I removed the base part of the case, leaving just the two parts covering each side of the phone, this eliminated the problem. It may be that cases of this type are causing, or making worse, flip phone screen cracking. I noticed the same problem with two third party covers of the same design, from two different suppliers, on two Samsung Galaxy Z Flip 3 model phones.