@Colw1 

Do not use the smart switch with the smart light bulbs, they can work independently, and smart control can be achieved without using them together. For example, a smart switch can control a regular light bulb connected to a non-smart socket. The smart bulb connects to a lamp base without smart module.

In your case, when the switch is turned off, it cuts off power to the bulb, disconnecting it from the router's WiFi network. As a result, the bulb goes offline, and the app may not respond immediately, which is why you still see the bulb appearing as if it's turned on.