It turned out that I had messed up my code in the update to Ionic 6.
The issue is solved by using Switch instead of IonRouterOutlet in App.tsx.
Actually, that was already in my code in Ionic 5 but I had changed it during the update.
As a result, this was not related to Ionic 6.
I am using Switch to navigate pages and IonRouterOutlet in one of the pages that includes several tabs.
I am doing so because I had issues when trying to use two IonRouterOutlet (one nested inside the other).
I am not saying it is not possible to do so but I was not able to make my app work that way.