Peter Gafert,

The problem comes from the fact that FakeBidirectionalRelationWorkUnit#generateData() invokes the relation mapper's mapToMapFromEntity() method where the following block of code is seen, include ToOneIdMapper.

	// If this property is originally non-insertable, but made insertable because it is in a many-to-one "fake"
	// bi-directional relation, we always store the "old", unchaged data, to prevent storing changes made
	// to this field. It is the responsibility of the collection to properly update it if it really changed.
	delegate.mapToMapFromEntity( newData, nonInsertableFake ? oldObj : newObj );

I see in FakeBidirectionalRelationWorkUnit we're providing oldObj as null and because this is a non-insertable fake relation, the value added for uniqueGroup_id will always be null as your test case confirms.

Reverting the above logic back to old behavior:

delegate.mapToMapFromEntity( newData, newObj );

This is the code used prior to adding support for @AuditMappedBy and this bypasses the issue but I'm not convinced that is the correct route yet.