PayPal checkout disappearing on forms

I’ve been using GravityForms for about a year now and have a recurring issue with the PayPal checkout. Frequently, the form will still appear, however the PayPal checkout disappears and there is simply a “Submit” button at the bottom of the form, with no PayPal checkout.

It happened again today, and I’ve narrowed the issue down (I think). If I remove the “Quantity” field from the form, the PayPal buttons reappear and work fine. The moment I add a quantity back in, the PayPal checkout disappears again.

Here’s the page with the live form on it:

Let me know if you have any ideas. Thanks!