Mapsui works on Android, fails on iOS?

I am trying to get a Mapsui MapControl to work properly in a .NET MAUI app. It's embedded in a ContentPage called MapPageSimple.

This is using .NET 9, Mapsui 5.0.0-beta.7, and doing this on VS Code on macOS.

It works perfectly on Android, the map shows up, it is lovely.

On iOS, if I set MainPage to a MapPageSimple instance, I see the map, yay.

But in my app, MainPage is set to a login screen. Once they log in, I set MainPage to a TabPage (well, a subclass of TabPage), and MapPageSimple is in one of the tabs. There, I can see where the control should be (I've set the BackgroundColor), but the map itself is not visible.

ChatGPT and I have spent hours fiddling with this, to no avail. Can someone please point me in the right direction?