This recipe will disable the HTML removal from email templates when using unsupported tags.

pmpro-disable-email-html-cleanup.php

<?php   // do not copy this line.
/**
 * This recipe will disable the HTML removal from email templates when using unsupported tags.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

add_filter( 'pmpro_kses', 'my_disable_pmpro_kses_for_email', 10, 3 );

/**
 * Disable pmpro_kses sanitization for the pmpro_email context.
 *
 * @param string $sanitized_string The sanitized string.
 * @param string $original_string  The original string.
 * @param string $context          The sanitization context.
 *
 * @return string The sanitized or original string depending on the context.
 */
function my_disable_pmpro_kses_for_email( $sanitized_string, $original_string, $context ) {
	// Only change if context is pmpro_email.
	if ( 'pmpro_email' !== $context ) {
		return $sanitized_string;
	}

	// Just return the original string and do not process it.
	return $original_string;
}