I narrowed down the conditions under which the problem happened to the following:
– When ApplePay is set in Stripe ( Enable Payment Request Buttons. (Apple Pay/Chrome Payment Request API)
– And the product is a variable product
– And I am looking in Safari browser (but not in Firefox or Chrome)
– And I am logged in
Then the Add To Basket button shows the -OR- which is part of the Pay with Apple Pay – OR –
In Safari/Privacy I disabled ‘Allow website to check for Apple Pay and Apple Card in’, and then logged out of my WooCommerce site and back in again and the problem went away.