I added a test (fudged 'skip') to S12-attributes/instance.t with commit https://github.com/perl6/roast/commit/dbf53cb4b1
AFAIU this inconsistency stems from using nqp::decont on attributes given to .new (in src/core/Mu.pm), but not doing so on attributes given to .clone. I added a PR (https://github.com/rakudo/rakudo/pull/443) which would make the added test pass.
