Guest checkout not working after update

I send clients invoices via WooCommerce, and they go directly to the payment page and make a payment (no carts, no subscriptions). I updated WooCommerce to 3.1.1 today (and also updated the rest of the site so everything’s current) and now the guest checkout functionality is no longer working. (Yes, Setting/Checkout/Enable guest checkout is checked.)

When you go to a payment page, it’s prompting “Please log in to your account below to continue to the payment form.” I see that in class-wc-shortcode-checkout.php (section starting at line 97) for “// Logged out customer does not have permission to pay for this order.”

I include my clients as Users in WooCommerce (with a Customer role) so that the emails go out and payments are properly recorded, etc. but I don’t want them to have to login to make a payment. (It’s never been an issue before.)

Can anyone provide direction on how to resolve this? Thanks.