Try to fetch related/linked Contracts - but result is always empty - why?

I try to get the contracts of an Opportunity. The Contract is seen and listed in the subpanel.

If i try to load related Contracts, i always get only an empty array.

/** @var Opportunity $quote */
$opp = \BeanFactory::getBean('Opportunities', $opportunitiesId);
$opp->load_relationship('contracts');
$contracts = $opp->contracts->get();

Any ideas what i am doing wrong? Working with Sugar 10.

Also tried "getBeans()" or similar functions. Result always empty Array :-(