To be clear, this is actually spec-compliant behavior. To quote the spec:

The conversion of all basic types is supported except for the following: Id attributes (including the
attributes of embedded ids and derived identities), version attributes, relationship attributes, and
attributes explicitly annotated as Enumerated or Temporal or designated as such in the XML
descriptor.

So this can clearly not be a bug. I'll leave it open for the moment as a feature request to see if others want to vote for it.