On iPhone, I find Life360 to be one of the most reliable solutions for presence.
This is completely normal, and is expected behavior as iOS devices will go into power saving mode when the screen is off to preseve battery life. As part of this normal power saving, the device will basically drop its WiFi connection (or at least become unresponsive to WiFi network requests.) iPhone WiFi Presence is most useful when used in conjunction with another method of presence sensing, and then combined with an App like Presence Governor.
You can also try using Locative on iOS with a virtual presence device on Hubitat, exposed via the Maker API.
