I tried adding hotspot user and specified uptime-limit to it and when I logged in that user account and after server minutes i turn off the router, my uptime is still 00:00:00 (not updated). It will update only when I logout from the hotspot.
The data is stored in RAM and not DISK. This is why it is reset after each reboot. You can search for scripts to save the data to disk or use User Manager.