Hi, I just faced a similar bug on my development website, I searched for days, and I finally find something :
Even if you don’t use Apple Pay/Google Pay (even if the box is unchecked), go to “Customize” and check if one of the 3 options is checked for the “Button appearance” section (Small / Medium / Large).
Selecting one of the options and then saving finally unlocked my problem.
I’m not sure it will be your problem, but give it a try.