while doing a merge, I got some PHP errors.
Seems like these steps fix the problem:
1. Change line 1215 of UnitOfWork.php to
$id = $class->getIdentifierValue($document);
it was ValueS before
2. in function _cascadeMerge starting in line 1464 of UnitOfWork.php
change the inner calls from:
$this->_cascadeMerge($relatedDocument, $managedCopy, $visited);
I didn't look deeper into ODM, just tried this to things and it seems to work now, so please excuse if it's bs ...