Have you uninstalled/reinstalled the Garmin app? Also try a restart on the phone before the reinstall.
My Vivoactive 3 Music started acting up in the same way a few months ago. I have Smart Unlock enabled on my Pixel 2XL, tied to my V3M, and noticed that it was going in and out of being connected OFTEN. I tried to work with tech support but they went for the hail mary and threw their hands up. I did the whole uninstall/install the app, same for the watch paring. I have a number of items paired to my phone, but rarely are two ever connected at any one time. I scoured the web and chat rooms and tried half a dozen tricks...nothing changed the connection dropping. Tech wanted me to get rid of all the paired devices and start re-adding to see if anything paired interfered. I declined...tech is just supposed to work, and it had been working until recently. I eventually gave up and went back to my Fitbit, which has no issues staying connected, no matter how many items I have paired. Sorry, I wish I had better news, but tech was unable to get my watch working, and getting rid of something else I use just to use the watch defeats the purpose of tech working together...how some can play nice together but others can't is beyond me. IF you are able to find a solution that does work, I would be all ears. Best of luck!