Autonumber field cannot be used in the formula field?

Autonumber field cannot be used in the formula field?

Tech support told me that Autonumber field cannot be used in the formula field. However, my formula field was working fine with autonumber. Did they change the change the setting?

My formula field is very simple. It shows "Last Name - autonumber."

ex) Smith - null

Of course, it shows "Last Name - null" when I save the record. But, when I edit the record, it surely showed "Last Name - autonumber."

ex) Smith - 5930

I wonder why thousands of records were working fine, and it stopped working recently.

Does anybody have a same experience using autonumber in the formula field?