Please go to SETTINGS > PREFERENCES > VOLUME SENSOR and adjust the setting.
To control the volume please make sure to move your finger on the outer ring (not in the area between the inner and outer ring). Additionally please hold your finger at the volume position you selected until the volume control disappears, and then let go. If you let go earlier the position can jump to another location due to the sensor believing the act of removing the finger was also a request to shift the finger.
In case of issues please unplug your ONE for a few minutes and turn it on again. Please also perform a SYSTEM CHECK (Settings > System > System Check > Enable > Check Now).