[BUG?] embedded records mixin does not use the customized `keyForAttribute`

I would like to customize the `keyForAttribute` function in such a way that some keys are serialized with as a different key(name). Defining this as an extension on the serializer works fine. However when this key is in a nested object it doesn't.

I wrote an integration spec reproducing the bug. Should I try to solve it, or is this expected behaviour?