Why the error "This server's certificate chain is incomplete" is only orange warning and the grade capped only down to B ?
Most "orange" warnings in the server test are for small errors or improvement needed.
When a server is reconfigured in this way, it's likely the owner didn't see it and will not experience it, but a lot of visitor will randomly encounter it. And it's not a small warning like mixed content, but the red scary message, and the inability to differentiate it from a real man-in-the-middle.
In my opinion, this error should be at least red, as it is an obvious misconfiguration.
Because of the effect of this misconfiguration and the difficulty for the owner to diagnose it,
I think it is important to highlight it, maybe with a grade capped to D or worst.