An iPhone turning off WiFi when asleep is not normal behavior. Mine does not do it. But there are reports other than yours of it happening (seen via a web search). Whatever the issue is is not Hubitat related. The issue is somewhere in your networking or phone.
- Make sure router firmware is up to date
- Set static DHCP reservation for phones
- Make sure Location Services is On and the Hubitat app is set to "Always"
- Make sure Background App Refresh is On and active for Hubitat app
iPhone network troubleshooting. These steps can fix weird networking behavior
- In WiFi settings forget the network. This is a simple procedure, you will have to just re-enter the WiFi password.
- If that does not work then you should "Reset Network Settings" from General>Reset menu. This resets all Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you’ve used before. The only downside to this procedure is that you have to re-enter all WiFi and VPN passwords.