This was fixed in MOBILE-3511, but after the Ionic 5 migration it's happening again in some cases.

For example, if you go to messaging using a tablet, a discussion page is loaded inside the split view. If you then click "Contacts", the ionViewWillLeave callback is called in the parent page (group conversations) but not in the child page (discussion page), it should be called in both places.

I also noticed a problem with ngOnDestroy, but I'm not sure if it will be easy to fix. Again, open messaging using a tablet, a discussion page is loaded inside the split view. If you then change the main menu tab (e.g. to My Courses) and go back to messaging, the child page is no longer visible (the placeholder is displayed) but the page wasn't destroyed so it is still fetching messages. If you then select another discussion, the page that wasn't visible is destroyed.