Ringtone Has Been Set to Maximum Volume on Its Own

Recently my ringtone has been set to maximum volume on its own. Does someone have the problem or tips? iOS 13.2.3.In the control center, the volume is set to approx. 1/3, but when I press the Volume Up / Down buttons it is at full volume and the volume also rings. If I make it quieter, after a while the volume will turn up again by itself, whereby 1/3 is still displayed in the control center.