The fix for HHH-9941 results in joins of the form:
leafsubclass lower0_ inner join
rootclass lower0_1_ on lower0_.id_=lower0_1.id1_ inner join
rootclass set1_ left outer join
TrivialClass set1_1_ on set1_.id1_=set1_1_.tcid left outer join
leafsubclass set1_2_ on set1_.id1_=set1_2_.id__ left outer join
nonleafsubclass set1_3_ on set1_.id1_=set1_3_.sid left outer join
leafsubsubclass set1_4_ on set1_.id1_=set1_4_.sid left outer join
mono set1_5_ on set1_.id1_=set1_5_.superid) on lower0_.id_=set1_4.parent

These joins don't work for H2.
If necessary, some logic would have to be incorporated in the Dialect.