We found the reason correctly.
The problem is caused by "WooCommerce Stripe Gateway" plugin setting.
In the setting page, you selected "custom" mode in "Payment Request Button Type" option like screenshot.
This option is related with the single product add to cart action directly.
The default option value is "Buy" like screenshot.
We set the default option in there. So everything works well now.
Please check again on your end and let us know.
Best Regards, C-Themes