Received an answer from Epicor support just now.
In short, if a pay to address is defined on any bank record for the supplier, the first pay to address it finds will override the supplier address for check printing, regardless of the payment method defined on the supplier master, or defined on the bank record. I think this is a bit silly because you might theoretically have a vendor that receives payments two ways in two locations, and it basically invalidates the one-to-many concept of having multiple bank records for this purpose. This was declared “working as intended”.
As for my specific issue, the reason the purchase point address was appearing in the VendBank address on the check was: my users had pulled a fast one and changed the paytoaddress on the VendBank record immediately before contacting me.
Hope this helps someone else out there.