I have Google reCAPTCHA enabled on my store. I have it on contact form, customer account sign-up, and checkout. v2 and v3 both work on contact form and customer account. Customers can send messages and sign up for accounts easily.
However, on checkout reCAPTCHA appears to block placing orders. It doesn't matter which, v2 or v3, the result is the same. Customer fills in shipping details then payment details. They click place order. The 3DS window comes up. 3DS authenticates successfully and the window closes. Then nothing happens. The place order button becomes un-interactive. The part that confuses me is that nothing appears in the console or the Magento logs to indicate what is going on. There is no JS error or application error.
If I disable reCAPTCHA for checkout, orders can be placed normally. So the problem is only on checkout and only when reCAPTCHA is enabled. The only possible clue I can see is that if I monitor the network tab in dev tools, the reCAPTCHA reload request does happen multiple times, it just doesn't seem to resolve.