Auto Unlock activated hours after manual open even though phone was home all day

So my dad manually unlocked and opened the door with a key at around 18:00 (weirdly this doesn’t show up in the activity log) then at around 20:00 the door auto-unlocked with my dad as the user in the activity log. Even though his phone was home all day and didn’t leave the geofence. My dad has the Smart Lock 4 without a bridge or Thread border router. How could that ever happen? Could it be because his phone was sitting in a location with a spotty connection to the lock? I also think the day before his phone battery died when he was outside the geofence, then charged it inside the fence. Could that be a problem? I don’t trust the Auto-unlock function anymore without using a door sensor.