Okay, I could solve the problem on my own, after discovering this in another topic:
Closing as won't fix: the problem is with the browser, not C2.
In Chrome for iOS, it correctly triggers 'On suspend' and 'On resume' when switching tabs, but not when returning to the home screen and going back (which Safari does correctly). Old Android stock browsers simply don't support the Page Visibility API which is the way the browser tells C2 if the page has been hidden, so it can't be supported. It should work in Chrome for Android though.
I solved the problem by adding this code snippet to the exported project:
document.addEventListener("deviceready", function() {
document.addEventListener("pause", simulatePageVisibilityApiHide, false);
document.addEventListener("resume", simulatePageVisibilityApiUnhide, false);
}, false);
function simulatePageVisibilityApiHide() {
document.hidden = true;
document.mozHidden = true;
document.webkitHidden = true;
document.msHidden = true;
var event = new Event('visibilitychange');
document.dispatchEvent(event);
}
function simulatePageVisibilityApiUnhide() {
document.hidden = false;
document.mozHidden = false;
document.webkitHidden = false;
document.msHidden = false;
var event = new Event('visibilitychange');
document.dispatchEvent(event);
}[/code:1n2qqp0m]