I see the wording 'reasonable best efforts' as another way of saying 'in good faith', which is presumed within a contract anyway. With advance ordering of supplies not yet produced then to mitigate against the risk of non-fulfilment, there needs to be 'work-in-progress' type reports as to how the project is going. There should have been an early contingency plan for any shortfall in expectation. For example, 'Party A shall notify Party B within X period of time, should delivery targets not be attainable'.
Either the EU were caught on the backfoot or AstraZeneca reneged. Or, of course, simpler than bad faith, it could just be an old-fashioned cock-up.