Uptime in phone settings change even with a simple reset, not just a factory reset and it was meant to give the data of the uptime since the last power off state. Btw for what purpose you need that data? Just count the months/years since you bought the phone.
Exactly what it was supposed to do🤷! How come something count that even after phone restart? It should be in the firmware then. There might me certain apps which can tell you that, by looking at the firmware clock (if that's even a thing).
