loose rivnut...

the stainless steel screw is cut to a slightly looser tolerance,,.. it screws in and pinches the face of the nut against the bracket, or the OD of the nut against the frame, before it has a chance to spin.

the other screw has enough drag, or a defect in the thread, to spin the nut before it gets pinched against the bracket or frame.

i've seen this before... your best course is to leave it alone to avoid ruining the nut further... although i've cleaned the frame/nut interface, greased the inner thread and the screw, then dripped one drop of superglue on the outer dia. of the nut before.. then quickly assembled the whatever.
Don't use quick setting super glue... you want a slower setup to allow assembly.. And anything past a minimum grease coating of the inner threads is a VERY bad thing.
this repair has risks involved.

best choice is: put it together, as is, with the stainless hardware, and leave it be.