Revert back to the original LearnDash course grid ribbons.

learndash-old-course-grid-ribbons.css

/**
 * This code is designed for those who have the Design Upgrade for LearnDash plugin installed.
 * https://wordpress.org/plugins/design-upgrade-learndash/
 * It also applies to the Pro version.
 *
 * These styles should be copied & pasted to a child theme or the "Additional CSS"
 * area of the WordPress Customizer.
 */

/* Add the old style of ribbons back to the course grid */
body .ld-course-list-items.row .ld_course_grid,
body .ld-course-list-items .ld_course_grid .thumbnail.course,
#ld_course_list .ld-course-list-items .ld_course_grid {
	overflow: visible;
}
body .ld-course-list-items .ld_course_grid .thumbnail.course .ld_course_grid_price {
	top: 10px;
	left: -8px;
	border-radius: 0;
	/* optional: if you want to set them to a specific width */
	width: 100px;
}
body .ld-course-list-items .ld_course_grid .thumbnail.course .ld_course_grid_price::before,
body .ld-course-list-items .ld_course_grid .thumbnail.course .ld_course_grid_price.ribbon-enrolled::before {
	display: block;
	border: 4px solid transparent;
	border-top: 4px solid #666;
	border-right: 4px solid #666;
	content: "";
	position: absolute;
	left: 0;
	bottom: -8px;
}
body .ld-course-list-items .ld_course_grid {
	border-radius: 0;
}