False alarms

Hi all,
Have had a good search but can’t find a solution to what’s happening here

So, only two of us in my house both with presence sensors on our keys. Very simple, both of us go out and the SmartThings alarm arms. When one of us comes home, it disarms. This has worked flawlessly for months until the last few weeks where probably 2-3 times a week one of us will open the front door and it will go into alarm but then pick up the presence sensor a few seconds later! Do we have any ideas on a resolution?
To add, since I’ve had this set up, we have had an alert that if someone knocks on the door and the door sensor picks it up, it sends us a message if we’re out. This never worked but left it there. Suddenly it has started working but there isn’t actually people at the door!!