UDO: "header line style" and User Field linked to a User Table

Hello,
I created a User Table A of "Master Data" type with a user field with Validation "Linked to Entities" linked to a User Table B.

i tried to create a UDO with the object registration wizard tool linked with the user table A .

in the UI Settings i've checked "Default Form" and i would like to use the "header Line Style";

after completing the registration of the object , I would expect the user field in a combobox with the values ​​linked to the table B.

Instead there is an EditText ( so I can not see the values ​​in Table B attached to the user field ).

why it doesn't work properly?

(if instead of selecting the "header line style" I select the "matrix style", the user field is properly presented in a combobox with the values ​​of Table B.)

sap version is 9.2 with PL 03

thanks in advance,

Michele