SharePoint Office Word integration

We use SharePoint Word integration to show and edit SharePoint properties in a Word document. Until now it works perfectly also with the users who are not so skilled in edit properties in the properties panel.
I encounter a problem if I want to edit a SharePoint property showed in the document with the text style 'hidden text'. It shows that the field becomes uncoupled from the SharePoint property in the document. Also the content of this field is deleted if the document is saved.

The questions I have are:

- Is it impossible due to technical reasons?

- Is it a bug?

- Do I something wrong and how can I solve this issue?

Regards, Peter