That's a massive if and just doesn't seem likely.
To take the PS5 analogy - say Sony sells one million to Argos, they then allocate how they'll get that one million to Argos. Three months later, Smyths orders one million. But there's then a production backup.
As a business, you don't take away from the allocation to Argos just to fulfil a later order from Smyths, harming both supply chains. In short, you don't breach one contract to lessen the severity of the breach to another, as it'd be unethical and result in two breached contracts.
It's not about choosing to honour one contract over another, it's about adapting to circumstance. What are AZ meant to do - cut off the UK from all vaccine supplies til the EU has caught up, when it's not the fault of the UK in the slightest?
And again, what motive - at all - does the EU have in deliberately not fulfilling the contract? They have literally none.