When the volume of Apple X SMS ringtone is turned off, it will become smaller by itself. The sound will be louder when setting, lower when exiting, and lower when re-entering.

This is because your iPhone X has turned on the gaze sensing function, which can be changed to the same loud sound.

When a mobile phone is talking, if you pick up the screen of the mobile phone or just use the mobile phone, iPhone X is detecting whether the eyes are looking directly at the screen through facial recognition, thus automatically adjusting the volume. IPhone X will automatically turn down the sound gradually. This function can be turned off.