Came across this here while searching for something, IIRC this was the old format used for user mentions before the BBCode version that is used now. It renders correctly on the profile post itself, but not in search results
Thank you for reporting this issue, it has now been resolved. We are aiming to include any changes that have been made in a future XF release (2.2.0 RC1).
Change log:
Strip structured text deliminators from search results