This was an oversight in https://github.com/NixOS/nixpkgs/pull/478539
that became apparent in https://github.com/NixOS/nixpkgs/pull/494416:
If there's a failing problem in Nixpkgs packages, CI will call
handleEvalIssue on it, but the problem error was missing a `.reason`.
Though even if it did have a reason, we need to do more to make sure
we don't break any code that uses it, so the new code uses the problem
kind as the reason, which happens to match with the reason for all
expected problem kinds.
Silvan Mosberger
764a7dbadf