I've come across this kind of behaviour in AD before. It looks like there may be some sort of 'undocumented' feature, examining search filters, and returning a null record set on anything with too broad a scope. managedBy probably doesn't have a medial index, so your search would be insanely slow, so AD returns nothing. There is some information on efficient filtering here

Just to verify that this behaviour originates in AD, you could try doing the same search with ldapsearch from the command line.

You might get better behaviour with (managedBy=cn=Administrator*)