Figured it out! In short, at least one binding to FieldNotify must be present in a widget, otherwise UWidgetFieldNotificationExtension is not added.
Why did I have widgets without bindings? Because I incorrectly used view models in some widgets having manually created conversion functions that were bound using a legacy binding system. A more sensible approach now with 5.3 is to have an intermediate variable that is also a FieldNotify that triggers a conversion function that is bound via the view model.