My Garmin Connect Android recently developed an annoying problem of not displaying the current ("live") HR (the big circle) while the HR history grath is readily available.
Killing and restarting the app didn't help. Restarting the phone did, but the problem recurred the next day. Not going to reboot the phone each day for sure.
So I tried this: I opened Garmin Connect app settings and from there cleared the app cache (NOT the app data!) and voila! All is well so far, let's hope the cure will persist, but clearing the cache is way less intrusive than the phone reboot.