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
Typing @Bro and then clicking on his name from the dropdown list didn't complete the tag.
Typing @Bro and then pressing enter did complete the tag
Typing @Bro and double clicking on his name did complete the tag
What's strange is that on the odd occasion I can click on @Brogan's name and it does complete the tag. However most of the time it doesn't and I can't find any rhyme nor reason as to why this isn't consistent.