I've noticed that during development, the only time that the code on my first screen's OnStart() property is executed is when I open the app for editing or viewing. It's inconvenient to have to re-open the app just to test my OnStart() code.
Please make OnStart() work on Previews or create a new kind of Preview that starts the app as it were used by some end-user.
Also, is it just me or the OnVisible property of the first screen doesn't get executed as I open the app? EDIT. It only doesn't on previews. I'm sorry.