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My application consists of five tabbed pages and nothing else. When it starts all of those tab pages are constructed and they remain that way until the application exits:

 public partial class MainPage : TabbedPage

 public MainPage()
 
 InitializeComponent();

 App.cardsPage = new DictionaryTab();
 App.homePage = new HomePage();
 App.helpPage = new HelpPage();
 App.settingsPage = new SettingsPage();
 App.phrasesPage = new PhrasesFrame(this);

 App.navHomePage = new NavigationPage(App.homePage);
 App.navHelpPage = new NavigationPage(App.helpPage);
 App.navSettingsPage = new NavigationPage(App.settingsPage);
 App.navCardsPage = new NavigationPage(App.cardsPage);
 App.navPhrasesFrame = new NavigationPage(App.phrasesPage);

 Children.Add(App.navHomePage);
 Children.Add(App.navHelpPage);
 Children.Add(App.navSettingsPage);
 Children.Add(App.navCardsPage);
 Children.Add(App.navPhrasesFrame);
 

In the constructors of four of the pages I am subscribing to a MessageCenter message. From what I observe these constructors are run only one time during the lifetime of the application.

It's my understanding that this is not the normal practice but in this case it allows me to update the pages so the results are immediately available when the pages are viewed later.

In this and only this case I would like to find out if it will be a problem. I understand that if the pages were constructed many times it could lead to many issues such as multiple subscriptions, memory problems etc, but in the case where they are tabbed pages that are opened just one time when the applications starts. Would that be a problem and if so what would the problem be?

You can, of course, subscribe here. But you will need to find a point in your page/app's lifecycle to unsubscribe. Else, this page might be alive forever, leaking memory.

A better option might be to subscribe whenever it appears and unsubscribe when it disappears as suggested in the comments.

You could also subscribe in the constructor and unsubscribe in the disappearing, but then the subscribe event will never happen again if the page is only instantiated once.