Rust on edge of standing seam metal roof.

What sort of environment is this installation in?

I was wondering whether you were in a salt or brackish water environment. It sounds like you probably are not. There are a lot of things that could have contributed and one is inferior raw material -- metal or coating issues. I am also unsure whether the panel was cut at that end or folded around but a fold may have been too tight or a cost could have been poorly done such as with a saw. There also could have been leak problems higher on the roof.

I would say this is not a common occurrence. It probably will require, at least, a site inspection from the manufacturer to determine what has happened.

How old is that roof?

Edges are almost never coated after cutting. In most cases, factory edges as you have said this is have very clean cuts which do not promote cut edge corrosion. In a site inspection, I would look for any sort of corrosive run-off that has been on the roof, or any reason moisture has been trapped against the edge of the roof. Barring those things, I would want to get samples of the metal to understand the quality of the metal and whether something went wrong perhaps with the paint pre-treatment. Of course, other things could be discovered along the way as well. Have you contacted the manufacturer? Unfortunately, there is just no way for me to diagnose this further without inspection.