This behaviour leads to that one could not see at first sight which type of message the notification was for, because the email subject is always "New Mention on...".
It would be more informating and appropriate to choose a email subject that makes clear at first sight which kind of mention the notification is about. (channel join, user disabling, mention, etc)
In my opinion a channel join is no mention in sense of a new message. There is a difference between a new message and other System activities and this should be clear withing the notification email.
Maybe ist reasonable to differ the email design a bit by choosing different colours for different notification types, so a user can see what the notification is about just based on the colours in the notification email.
see: https://github.com/mattermost/platform/issues/2312#issuecomment-196896155