Shouldn't this be fixed upstream? Or is it abandoned?

It will not be fixed upstream because this is a part of the expected output of this linter.

I guess I mean why do the linter expect relative/short path on Linux but full path on macOS. That's just inconsistency to me.

But yes I agree the linter expects to show which file and line is affected and no matter the format it will be duplicated info in golangci-lint which isn't needed so 🤷