I had a leak at the egr pipe on my manifold once. I couldn't repair it so I got a used to replace, but it was caused by the
pipe rusting out, not anything actually happening to the manifold. I think you'd have to really abuse the manifold to hurt em.
I've even drilled out the studs on my driver side one and put in new ones. I'm pretty sure I put them in a few turns tighter than it needed to be and still no issues.
I'd look at the donut gaskets in your exhaust, if a bolt rusts good enough the spring can break it off if you hit a good bump. It would make it loose enough for a good size leak and still look like it is together.