For the last couple of months I had intermittent problems with bluetooth connection between my Android phone (Samsung Galaxy A3 2017) and the FR735XT. It would work ok, then suddenly the watch would disconnect and would refuse to reconnect automatically, unless bluetooth, internet and location (all of them) were toggled off and on. Then the watch would reconnect but only to the next disconnection, when I would have to repeat the procedure. This would continue for days, sometimes weeks, until all of a sudden connection would come back to normal regime for no apparent reason. And after another couple of days or weeks all of this would repeat again.
For months I was baffled by this issue. I could not find anyone else reporting similar symptoms, I could not attribute the issue to any changes made to either my phone or the watch. It would just work for weeks, and then get broken for another couple of weeks.
But now I think I have pinpointed the reason to another app on my phone (Bluetens) interfering with bluetooth connection. It's an app for remote control of a medical device over bluetooth, and so it appears it does something very peculiar to bluetooth on the phone. I'm not 100% if this is the reason, because the issue is hard to repeat, but I managed to resolve the reappearing connection issue twice so far, by killing the Bluetens app, which for some reason keeps running in the background.
I don't know if it's the app's fault, or if it's Garmin Connect app's fault, or if it's a deficiency of Android, but at least now I have an ad hoc solution in case it happens again in the future.