I can reproduce this error if I use Spring 5.0 + Hibernate 5.2.12.
I cannot reproduce it with only Hibernate 5.2.12 without Spring.
(And I don't know whats is the difference between the two)

Strangely the converter is instanciated in both cases.

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According to the JPA 2.2 specs

The Convert annotation should not be used to specify conversion of the following: Id attributes
(including the attributes of embedded ids and derived identities), version attributes, relationship
attributes, and attributes explicitly annotated (or designated via XML) as
Enumerated or Tempo-ral. Applications that specify such conversions will not be portable.

http://download.oracle.com/otn-pub/jcp/persistence-2_2-mrel-eval-spec/JavaPersistence.pdf