Presence detection iPhone 6s Plus

I have been a SmartThings user for a few years. My wife had an iPhone 6s Plus that was flaky with presence detection. Sometimes it would report she was home when she was not and sometimes it would report she was not at home when she was. I dealt with this quirk by having her open the app which would correct it. Fast forward to a few months ago. She got a new iPhone XS Max and I got her old 6s Plus and got rid of my iPhone 6s which presence detection worked great… I wiped the 6s Plus and restored my backup for the 6s from the cloud. The presence detection is still flaky even after the new iOS update.
This is important because I have several routines for the HVAC and door lock that rely on presence detection.
Could the phone be at fault? Presence detection on my 6s was always spot on so I assumed when I restored the backup from that phone to the 6s Plus everything should work fine.
I can be at work 30 miles away and the app will say I am home. All I have to do is open the app and a few seconds later boom I am not home.
Any ideas are appreciated. This has been an ongoing problem with the 6s Plus for a few years.