According to the Hibernate documentation, LockMode.FORCE is "similar to UPGRADE except that, for versioned entities, it results in a forced version increment". The version increment often produces undue stale state exceptions under databases that support isolation levels above READ_UNCOMMITTED, i.e. all except HSQLDB.
Under HSQLDB, LockMode.FORCE provides a simplistic work-around the lack of isolation. Unfortunately it does not constitute a real solution since dirty reads can occur anywhere else, causing all kinds of "weird" assertion failures. Every test case that exercises concurrency has been excluded from the HSQLDB test suite due to its unreliability.
Therefore, no reason remains to keep the default parentLockMode at FORCE.