[TYPO3-mvc] backend flexform is overwritten while extending fe_users

[Domi]

Hello,

I overwrite somewhere my TCA of my fe_users in the backend but not sure 
where else to search. I extended fe_users with several different users 
such as:

ExtbaseFrontendUser
   - IntranetUser
     - Agent
     - Client
     - CustomerService

I did the whole setup of my extension with the extension builder. If I 
click on any frontend user All I see is just the tab of their specific 
fieldset.

Why is not the tab extending the model of extbase frontend user, but 
seems to overwrite it? Has someone a keyword I could look after, how to 
find the problem? The file ext_table.php looks fine to me.

Thanks for any hints

Dominic