Vivoactive

The separation between the watch and the headphones

The Bluetooth receiver's placement will differ between brands and types of headphones, for example, in one or both earbuds or in the control module.
Possible Solution: Wearing the watch on the other wrist may assist to relieve the audio cutting off issue.

Being indoors vs. being outside

When you're indoors, the Bluetooth signal might bounce off the walls as data travels between your watch and headphones, resulting in greater reception than when you're outside.

Body Interference

Some versions may allow you to wear the headphones so that the Bluetooth receiver is not pressed against your skull. In general, keeping the receiver away from your head or torso improves Bluetooth reception.

Outdated software

Ascertain that your Bluetooth Headset is using the most up-to-date system software. If you need help, contact the maker of your headphones.
Connect the Garmin device to a computer and install Garmin Express to ensure it is running the most recent software version. Once you've added the device to the programme, Garmin Express will automatically look for and install any available updates.

Headphones connected to a different device

If you link your headphones with both your watch and your phone, the phone can "steal" the Bluetooth connection to the headphones, causing the headphones to detach from the watch. In this scenario, unplug your phone's headphones or temporarily stop Bluetooth on your phone while listening to music through your watch.

Other considerations

Low-battery-charged headphones may cause the connection to drop.
Some headphones have more sensitive receivers than others, and those with the most sensitive receivers will function best with your Garmin device.

Hard Reset GARMIN Vivoactive 3

Start by pressing the Power button to wake up the screen.

Now touch and hold the touchscreen to open the menu.

Once the menu appears on the screen, scroll down and find Settings. ...

Next, scroll all the way down and tap on the System.

Now scroll down and select Restore Defaults.

https://www.hardreset.info/devices/garmin/garmin-vivoactive-3/

This should fix if any issue with device in minor sense, However if you feel that is still remain the same after reset.. contact warranty support.. chance of an issue with product firmware/software/hardware.