I would guess that is more about rust reforming than anything. I can't Imagine there is too much flex going on back there unless you were rocking some serious HP before. I think regular body filler is a little more forgiving in the flexibility aspect. The "all metal" fillers are a lot more rigid. I'll use something like all metal filler just because that's what I've always done on rust repairs.
On my 1st Dart 30 years ago, I did a super fast weekend rust cover and paint the day I bought it. Just used expanding foam into wire brushed and rust stop - rust holes in the quarters and rocker panels. Used All Metal filler. It still didn't have any rust bubbles 4 years later when I took it apart to restore it. Then I stupidly sold it for parts while I was away at college.
