The OnTap event for TTMSFMXNativeUIView does not appear to work on iOS in Delphi 10.3 Rio (it may work in other versions). It's fairly simple to reproduce: put a TTMSFMXNativeUIView on a form, and put some code in the OnTap event, and the code is never executed.
The native iCL components do not interact with the FMX layer, therefore the OnTap, and many other events, are not triggered. Please use the OnClick event instead. Alternatively, you can handle taps manually by adding native code to create and manage a tap gesture recognizer.
OnClick is not published, however I can assign a handler at runtime, but it does not work, either.