I had issues with that since my phone's Wifi chip seems to have powersaving in it also, which means if my phone is screen-off in my pocket, it's not necessarily 'connected' to my router as far as the router is concerned. It effectively reconnects sporadically to grab push notifications or when I turn on the screen, but otherwise is mostly sleeping. I wasn't able to come up with a presence detection system more reliable than the one HomeAssistant uses, which is "is $my_phone inside $gps_fence"
What kind of granularity do you need? When I had that running I set the timeout to an hour which seemed to be enough to cover those disconnection periods. Of course, it meant that it takes an hour for Home Assistant to actually notice that my phone is gone but that was OK for my use-case.
