How to print name of Payment Method in Commerce 2 order receipt email?
This patch (https://www.drupal.org/files/issues/2019-02-26/order_receipt_payment_gateway_and_instructions.patch) from this thread (https://www.drupal.org/project/commerce/issues/2884497#comment-12988057) fixed my issue. It required patching commerce_order.module and commerce_payment.module, as well as adding the display code to the order email receipt twig.