TinyMCE editor – new lines not rendered on front end

As mentioned in the comments, this is a lack of wp_autop() and other formatting functions that get applied to the the_content filter. The editor does not save <p> tags at all, but later on line-breaks are converted into paragraphs via wp_autop().

For meta content, I like to recreate the default filters. I do this because some plugins add things via the the_content filter, Once I had apply_filters('the_content', $your_meta_content); for all my meta fields and ended up with 20 sets of social sharing buttons.

/* 
 * Recreate the default filters on the_content
 * this will make it much easier to output the meta content with proper/expected formatting
*/
add_filter( 'meta_content', 'wptexturize'        );
add_filter( 'meta_content', 'convert_smilies'    );
add_filter( 'meta_content', 'convert_chars'      );
add_filter( 'meta_content', 'wpautop'            );
add_filter( 'meta_content', 'shortcode_unautop'  );
add_filter( 'meta_content', 'prepend_attachment' );
add_filter( 'meta_content', 'do_shortcode');

Then where you actually wish to display your new data, you should write the following:

$description = get_post_meta($post->ID, 'sdm_description', true);
echo apply_filters( 'meta_content', $description );