TNEW v7 Payment Plans and Payment Express

We've upgraded to Tessitura v15.1 and have been on TNEW v7 for some time. We're in the process of switching over to Payment Express, but I cannot for the life of me get Payment Plans working in our TNEW Test site. I've run multiple paid in full test transactions through the test Payment Express hosted payment application, but each time I try putting a Payment Plan on the order I ultimately end up back on the TNEW payment page with an error message at the top of the screen that states "Your payment could not be processed. Please review your payment information and try again." (/cart/payment?errorMessages=HOSTED-PAYMENT-FAILED-WITH-REFUND) This happens after I've received notification from the Payment Express app that my test card was approved. If I take the Payment Plan off the order and try the same card again, the order completes without issue.

I've checked both the Hosted Payments and Payment Plans documentation multiple times and can't find any steps I've missed. I have a ticket in with RAMP Support as well, but thought maybe someone else in the collective user space might have encountered this same problem.

Thanks in advance for any guidance!

Sara