Im,possible for this to be a bug. This is not documented to work and even JPA says it is not expected to work:

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. Auto-apply converters will not be applied to such attributes, and applications that apply converters to such attributes through use of the Convert annotation will not be portable.

You are requesting an improvement.