installment payments failing with instant modal checkout

Have you experienced this?

If you go here, the 3-pay button shows the 1-pay version:
https://learn.parishealingarts.com/metabolic-health-group/

But if you go here, the product page has both payment options, and they work correctly:
https://learn.parishealingarts.com/item/metabolic-health-reset/

 One thing I notice: the instant modal button has item_id=1 for both payment options, and no other signifier of payment type.

Yet I have very carefully chosen the 3-pay variant on the 3-pay button. I've created that button from scratch several times.

Pro version 1.3.10