I can reproduce it.
When users have a space in their name, it isn't inserted.
Try @
Mike and then clicking Mike Buttery
/hmm, in this post it didn't work, but when I try it in a new post in the editor below it does work with Mike Buttery.
/trying a third time and it's not inserting again ==> Think it's related to the space in the name
Weird.
Running Chrome on Win 7