Hi! Welcome to the Samsung Members community. As it's working normally with other audio types, this suggests a setting within YouTube Music—but I'll give some things to check with the device.
1. Swipe down from the top of the screen to open the Quick Settings panel.
2. Tap Media under the Quick settings icons.
3. Select the device you want to use from the list of connected Bluetooth audio devices.
Using the Settings app:
1. Open the Settings app.
2. Tap Connected devices.
3. Next to your device, tap Settings.
4. Tap Audio switch.