Thanks for letting us know about the sample bug. Please add this to the AddEntryPropertyModel constructor:
this.ValueType = typeof(string);
That will get the ValueToString back to Value conversion working, which will allow the text entry and "+" button to show up again. We also will be updating our code for the next build so that line isn't needed in this sample since it will assume if no ValueType is specified, the ValueType is Object, but it's still a best practice to specify the ValueType.
The other issue is that since PropertyModel changed to inherit FrameworkElement in v2020.1 so that data binding could be supported in XAML, it took over the Parent property. In the AddEntryPropertyModel.AddChild method, change this line to get the IDataModel.Parent instead of FrameworkElement.Parent:
var rootModel = ((IDataModel)this).Parent as IRootModel;
Those two updates should resolve the problem.