With https://hibernate.atlassian.net/browse/HHH-16281 the disassembling of OneToMany collections doesn’t work properly and renders the identifier object as a collection of objects ( instead of provided identifier type).
Actually the identifier seems to be disassembled twice, first on NonAggregatedIdentifierMappingImpl.getIdentifier for the Entity object and secondly on AbstractEmbeddableMapping.dissamble of the identifier object.

As result, an IllegalArgumentException is thrown when setting the identifier values:

Caused by: org.hibernate.property.access.spi.PropertyAccessException: Error accessing field [private java.lang.Long com.example.demo.domain.issue.DoubleId.id] by reflection for persistent property com.example.demo.domain.issue.DoubleId#id : [Ljava.lang.Object;@cc46555
{{ at app//org.hibernate.property.access.spi.GetterFieldImpl.get(GetterFieldImpl.java:48)}}
{{ at app//org.hibernate.metamodel.mapping.AttributeMapping.getValue(AttributeMapping.java:59)}}
{{ at app//org.hibernate.metamodel.mapping.internal.AbstractEmbeddableMapping.breakDownJdbcValues(AbstractEmbeddableMapping.java:594)}}
{{ at app//org.hibernate.metamodel.mapping.internal.NonAggregatedIdentifierMappingImpl.breakDownJdbcValues(NonAggregatedIdentifierMappingImpl.java:299)}}
{{ at app//org.hibernate.metamodel.mapping.internal.OneToManyCollectionPart.breakDownJdbcValues(OneToManyCollectionPart.java:98)}}
{{ at app//org.hibernate.metamodel.mapping.ModelPart.decompose(ModelPart.java:194)}}
{{ at app//org.hibernate.persister.collection.OneToManyPersister.applyDeleteRowRestrictions(OneToManyPersister.java:640)}}
{{ at app//org.hibernate.persister.collection.mutation.DeleteRowsCoordinatorStandard.deleteRows(DeleteRowsCoordinatorStandard.java:97)}}
{{ at app//org.hibernate.persister.collection.OneToManyPersister.deleteRows(OneToManyPersister.java:194)}}
{{ at app//org.hibernate.action.internal.CollectionUpdateAction.execute(CollectionUpdateAction.java:83)}}
...
Caused by: java.lang.IllegalArgumentException: Can not set java.lang.Long field com.example.demo.domain.issue.DoubleId.id to [Ljava.lang.Object;
...
{{ at java.base/java.lang.reflect.Field.get(Field.java:425)}}
{{ at org.hibernate.property.access.spi.GetterFieldImpl.get(GetterFieldImpl.java:44)}}
{{ ... 115 more}}

My sample project uses Lombok / Spring Data.

• a Content class with inverse OnToMany reference:

  @Entity
  @Getter
  @Setter
  public class Content {
  
      @EmbeddedId
      private PkComposite id;
  
      private String name;
  
      @OneToMany(cascade = ALL, orphanRemoval = true)
      @JoinColumn(name = "ID_CONTENT", referencedColumnName = "ID")
      private List<Detail> detailList = new ArrayList<>();
  }

• its composite ID class:

  @Getter
  @Setter
  public class PkComposite implements Serializable {
  
      @Column(name = "ID")
      private Long id;
  
      public PkComposite withId(Long id) {
          this.id = id;
          return this;
      }
  }

• a Detail class using a composite key defined with @IdClass:

  @Entity
  @Getter
  @Setter
  @IdClass(DoubleId.class)
  public class Detail {
  
      @Id
      private Long id;
      @Id
      private Long id2;
  
      private String name;
  }

• its composite key:

  @Getter
  @Setter
  public class DoubleId implements Serializable {
      private Long id;
      private Long id2;
  }

• the test that fails with the exception detailed above:

  @Test
      @Transactional
      public void issue4_disassamble_idclass() {
          // create some data
          Session session = entityManager.unwrap(Session.class);
          session.createNativeQuery("insert into CONTENT (ID, NAME) values (200, 'NAME for CONTENT 320')").executeUpdate();
          session.createNativeQuery("insert into DETAIL (ID, ID2, NAME, ID_CONTENT) values (300, 400, 'NAME for DETAIL 300', 200)").executeUpdate();
  
          // Commit transaction
          TestTransaction.flagForCommit();
          TestTransaction.end();
          TestTransaction.start();
  
          Content content200 = contentRepository.findById(new PkComposite().withId(200L)).orElse(null);
          Assertions.assertNotNull(content200);
          Assertions.assertNotNull(content200.getDetailList());
          Assertions.assertEquals(1, content200.getDetailList().size());
  
  //        content200.getDetailList().clear();
          content200.getDetailList().remove(content200.getDetailList().get(0));
          Detail newDetail = new Detail();
          newDetail.setId(301L);
          newDetail.setId2(901L);
          newDetail.setName("NAME for DETAIL 301");
          content200.getDetailList().add(newDetail);
  
          Content content200Upd = contentRepository.save(content200);
          Assertions.assertEquals(1, content200Upd.getDetailList().size());
          TestTransaction.flagForCommit();
          TestTransaction.end();
          TestTransaction.start();
      }

I have attached the tests logs. If needed, I can share my sample project.