Thanks Adam, the pages are using the built-in WordPress block called:
“Details
Hide and show additional content.”
I guess this uses Ajax under the hood, but the only adjustable settings for the block in the WordPress sidebar is an “Open by default” toggle switch.
I created a test page without the “Details” block here:
https://biodo.eu/contact-us-test-page/
The Stripe payment field shows up on my iOS device for this test page.