A customer at my shop has a z Fold 3 and apparently debris or lint or something got into the hinge and it quite doesn’t open all the way. IbreakUfix told him to wait awhile and move it back and forth and eventually the debris shoudl work loose instead of paying to have it fixed.
This is a big concern with me. Even with a case on it your phone still gets lint on it from daily use and being in your pocket. With the phone not being dust resistant is lint causing any issues when getting into speaker grills or the hinge which is common to get lint in the speaker and port areas of phones.