Volume goes down when tracking
If your volume gradually lowers after a while, when the tracking is running, the option Turn off when sleeping is enabled. This feature lowers the system-wide volume, so it affects even music played outside Sleep.
Go to Settings → Sleep tracking → Lullabies → Turn off when sleeping.
Extend the minimal playback time (5-90 min).
Or disable the feature completely by setting it to Use current device volume.
Tags: lullaby, tracking, volume