When multimedia filters are enabled some media players (at least YouTube, Vimeo) try to transform links to those sites into iframes so, if you insert a link to those sites on a forum post with that filter enabled, that iframe content isn't shown on email notification content because email server/client sanitizes the iframes.
Steps to replicate this with YouTube player (it also happens with Vimeo and the YouTube setting of the VideoJS player):
- Enable the multimedia plugin
- Enable the YouTube media player and put it the first of the list to ensure it gets applied
- Create a test course and enrol a user into it
- Create a forum with forced or auto suscription
- Add a new topic and insert a link to some YouTube video in the message
- Check that on Moodle it gets converted to a iframe
- Check that on the email notification sent to the user the iframe isn't shown
Most modern e-mail providers usually sanitize iframes, so to avoid this happening I think that multimedia filters that use iframes may shouldn't be applied to notifications sent by email.
I doubt if it should be done also with players using HTML5 <video> and <audio> tags like VideoJS as they are still supported only by a few email clients.