How do I stop the game restarting when it is still open but not active?

When I open my app it starts as it should but when I select another app and spend some time in the other app and then, when I come back to my app it restarts. I am not closing the app, in other words it is still on "hold" (I am sure there is a official name for this).

How do I prevent the restart?

Here is an example of what I mean by on "hold"

That's generally a feature specific to your phone or operating system.

For example, Xiaomi phones are very aggressive in closing apps that are idle for a certain period of time to save system resources.

You might be able to find options in the power management settings of your device.

dontkillmyapp.com

That link is pretty great.