neo-SSPX 
Certainly there are chapels where this sort of thing does not regularly take place. Perhaps you have the blessing of attending one?
It occurs when the top does not have fabric which would normally cover the entire back, causing some amount of the back to be bare.
Thankfully, such attire is not seen in Mass, not even at the neo-SSPX if I have to go to Mass there. SSPV Mass, I’ve never seen it, either. I’ve been to resistance Masses as well, and only one can I recall a couple dressed not really immodestly, but too informally. The Mass was at a hotel and they got talking with the priest, were interested and he invited them to Mass. An extra veil was procured for the woman and she wore it.