Your AC pushbutton control unit could be having a problem, or you have another problem elsewhere.
I would search on this and see what you come up with. I think others have had this issue before.
Hope that helps in your search for proper working AC.
I'm not a tech, but I think I remember reading about similar issues with faulty ac controls..
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.