Dan is exactly right here, your contract with Monzo is to provide the services you paid for. When you pay via PayPal then you’re buying the ‘credit’ from PayPal to buy something from somebody else. Therefore the service you paid for has been delivered.
This means that we have no chargeback rights for PayPal transactions for goods and services disputes. As raising a chargeback which you lose costs money we won’t raise chargebacks in situations where we’re certain to lose.
In addition the chargeback scheme is a voluntary one. Banks are under no legal obligation to raise a chargeback and it’s at their discretion.
I never pay via PayPal for this very reason.
Edit: the easy way to think of it is similar to buying a gift card. Once you’ve bought the gift card then the service has been fulfilled. Even if the merchant goes bust before you can spend the money then you can’t claim it back.