As a part of PayPal requirements for recurring subscriptions, your PayPal account will need to have an instant funding source (such as a credit card) attached to the account. If they do not have this, then you will not be able to utilize PayPal as a funding method on Patreon.
With that in mind, how things work with PayPal transactions is that they always first attempt to use your PayPal balance, and then, if that has insufficient funds, they use whatever funding instrument (e.g. a bank account, or a credit card) you selected when going through the PayPal pop-up flow on Patreon.
This whole flow is dictated by Paypal, and isn't something that we can change currently. Down the line, we hope to provide even more options for patrons to pay creators on Patreon.