Thank you for the fast response! 👍

I use an H2 database in my reproducer, but you are right. When adding

quarkus.hibernate-orm.fetch.batch-size=-1

in the application.properties file in order to deactive the batch loading (as a workaround), the error does not appear anymore.

But then the problem is, that now the relation objects are not loaded at all. When I access the getter method of the relation the select query is executed, but I don’t get any result.

The query

Hibernate: 
    select
        e1_0.a_id,
        e1_0.id 
    from
        EntityB e1_0 
    where
        e1_0.a_id=?
2023-07-26 10:18:08,264 TRACE [org.hib.orm.jdb.bind] (main) binding parameter [1] as [VARCHAR] - [org.acme.types.StringWrapper@49fb693d]

When I select the entities directly, it works as expected. So I assume, that the user type in general works.

Example query (for my reproducer)

public void findEntityAByField() {
    final var qlString = "SELECT a FROM " + EntityA.class.getName() + " a WHERE a.id = :idField";
    final var query = em.createQuery(qlString, EntityA.class);
    query.setParameter("idField", new StringWrapper("ID_A_001"));
    final var resultList = query.getResultList();

    assertEquals(1, resultList.size());
}

The query looks like this

Hibernate: 
    select
        e1_0.id 
    from
        EntityA e1_0 
    where
        e1_0.id=?
2023-07-26 10:18:08,452 TRACE [org.hib.orm.jdb.bind] (main) binding parameter [1] as [VARCHAR] - [org.acme.types.StringWrapper@5fe4848]

Here I get the entity as expected. As I said, because of this I think the user type works in general.

I created a new reproducer for this: https://github.com/timonzi/hibernate-user-type-for-relation-disabled-fetch

Am I missing something or is this (another) bug?