I've 935 with latest fw + iPhone 12 mini (but it happened in the past with iPhone 7 as well). What I observe is that watch is connected to the phone but synchronization doesn't happen.
Watch reports: connected - sync in progress
App reports: red exclamation mark next to watch. Forcing sync at that time rarely helps.
W/A is to disconnect watch and connect again (not pairing, just disconnect) or cycling BT power on the phone.
Could you please fix it? Seems like a simple issue with connection state machine. Also if BT connection gets stuck, watch could switch to WiFi. I had 920 which always use wifi when I returned home, now 935 seems to prioritize slow BT over WiFi so Wifi is almost never used.
Thanks,
Mikolaj