These weren't made with the greatest care to begin with. Could be retrograde or just sloppy like this one.
Marcian monogram coins with a Z are scarce, but not unknown. Maybe it was some sort of officina mark? Maybe the engraver was illiterate? Maybe they are all just barbarous? We may never know
Thanks for your help. I saw a monogram with the "z" but it also had "+" sign at the top which this one doesn't have.