Hmm, another 100 iterations without a problem. Not sure whether it's related to back-mdb or whether this may happen occasionally with other backends too. Trying more...
Looks like a simple timing dependency in the test script. slapcat of the DB in your testrun directory shows that the correct value is present. The refint overlay does its work in a separate task, so there's no guarantee it will be done by the time the original modrdn request finishes.
We could add a SLEEP in the script here but it seems to be such a rare occurrence I'm inclined to ignore it.