RBs shouldn't have mirror slap if the bumpers are properly installed n sized.. a common DIY mistake. There is a strip of foam under the mirror.. that stops the other flap dead.

2 square bumpers (one is larger than the other, takes up the slant of the mirror), just under the front top lip of the mirror box...those are to stop the mirror dead n keep it from reverberating.

If you had the bellows off, this becomes more apparent.. but not many get that oportunity to see it function n just replace em as if they are light seals.

But whatever you did to cure your problem is the correct solution.. everyone has a differet way of doing it and the end result, the pictures, is where it counts most.