Similar issues with a tabbed interface (derived from the starter template). onPageWillEnter() does not fire for initial occasion(s), but onPageDidEnter() does. Not for all tabs. Just for one in an array of five or so. OnPageWillEnter() does fire once I have selected things in the tab and/or other tabs.
So still investigating when this all occurs. (btw, I thought I was on beta, but I am still on alpha 53 - so need to upgrade. maybe that helps).
I have figured it out - as Tom said, it occurs on tabbed project - what happens actually is that the TABBED page is actually the one that the onPageWlllEnter (or DidEnter) is being called on - and it DOES NOT “bubble” to the ACTIVE page (the current tab) - I don’t know if it is a bug or by design - but now - as I have implemented the method in the TABBED page, I can easily “notify” the active tab page about it having the focus again, and invoking any logic on that page manually.