Strange Java customization Behaviour

Using the attached java class customization (it is a simplification, developed to show this strange problem)

I have the following problem: when you edit attributes of a document, and click on the button in the attribute editor of this customization, you can correctly create a new Masterdata as expected by this customization.

But if you use the custom button during save command of cad integration, you can still create a new Masterdata, but you cant't write values in the Dialog. Values are editable, for example if you have a combo box attribute you can select a value, but you can't write with keyboard.

then, click the button again. The second time everything work as expected...

I can't understand if this is my fault, Model Manager bug or Java Bug.