I agree that to not know your name at all and to answer to 'Oi' or 'You' or 'Little F---er', or to pay more attention to tone of voice than any particular name is the tip of another iceberg altogether.
And, tbh, that's the only kind of 'not knowing your own name' that I would find at all worrying as a teacher. Answering to a pet or nickname and not knowing the full one is not something I would worry about - whether it is of the 'Ptolemy Ned' or 'sexy babe' variety should matter not one jot.
Also, it is most concerning as part of a general pattern of emotional and personal neglect - again thinking of particular children but obviously not describing them in too much detail on this forum - such as not knowing how to give / receive a hug or other affectionate gesture, not being fed before or after school, clothes being absent / dirty / wholly unsuitable for colder weather, no care for teeth or hair or skin cleanliness. As a 'stand alone' absence in a child's life the knowledge of a name is, after all, a fairly simple thing to remedy ... other forms of neglect and abuse are often much harder to tackle.