Puzzling problem with action mapping in UE 4.27

I have 2 action mappings where “Z” is mapped to a zoom function and “Ctrl Z” is mapped to a super-zoom function. Both mappings work as expected. HOWEVER, If I press “Shift Z” (which is not mapped to any action) it invokes the “Z” zoom function. I have checked through every INI file in my project, and nowhere is “Shift Z” mapped. Does anyone know what’s going on here? and how to correct it?