Sequentia

I flipped the combo card and found my full middle name there! False alarm

I came to the US on K-1, got married and submitted AOS/EAD/AP with my maiden name as my new middle name, and husband's surname as my new surname filling in all the forms this way. All correspondence (NOA) from USCIS addressed me with my new full name incl. full new middle name. Then, the combo card came, and I only see the initial of my middle name there. Yesterday, we were buying a car for me. The dealership only considered my initial in the papers as I couldn't show any legally valid proof for my full middle name. I now expect the same to happen with SSN, driving license, employment contract. What my full legal name is now? What do I do if I still want my full maiden name to be my middle name? Such a happy day to get the combo card, but I'm totally confused.