Amazing how some say apply bondo over bare metal only and others say
over primer is fine.
I'm not entirely sure of the answer but I had to use a lot on a 68 Cougar
back when I was 19 ( 30+ years ago).
I only used flat Autozone primer - not epoxy - and and to this day there are only 2 VERY small hairline cracks that ever developed in the polyurathane paint.
I'm wondering if the bondo was applied over rusty metal or the rust naturally developed later.
And the always present possibility that you are looking at the "thin" edge of a much deeper pile nearby.