This is a fundamental core Samsung Android issue that’s been around for a while and it’s causing Samsung Galaxy phones to have these unreliable geofencing behaviors.
Even with the “battery saving” and “doze” features turned off, the core OS memory management system in the Samsung Android kills background processes regularly, even if there’s plenty of memory left in the system. This is a well known fact amongst the Android enthusiasts at sites like XDA. One example:
I’ve experienced your exact issue with various Samsung Galaxy phones (S6 edge, S7 edge, Note 7) over the years and not just with ST mobile presence. Geofencing in general on recent Galaxy phones are never reliable because the OS memory management system is constantly killing background processes, such as Life360, ST, IFTTT, etc., no matter what setting you have set for “battery saving” or “doze.” If you root your phone, I believe you can reduce this by messing with the core OS but I never tried.
I recently moved to iPhone 7 Plus to replace my exploding Note 7 and so far I’ve yet to have a single failure of any geofencing app. Zero.
My wife’s Note 7 (waiting for replacement) still “leaves” home every few days at random times on both the ST and Life360 mobile presence.
Not sure which Galaxy you have but try this to see if it’s the core OS issue like mine or specific app issue.
Setup multiple apps, Life360, ST, IFTTT, etc. with the same geofence (i.e. your home). Set them up to notify you when you arrive/leave that geofence. If they all report the same incorrect location then you have a core OS issue as I described. Otherwise, if they report different things then it’s likely specific app related.
Good luck.