No bug. Look at second paragraph where I discuss when Bonus will be highlighted. Haunting bonus needs 16% Clearcast for 40% more damage.
The bonuses will highlight on the character you are controlling in the arena. After or during an arena match, go look at your gear with the bonus and it will be highlighted/activated. If you are using a bonus on a hireling in 2v2, the activated bonus will not show but it will be working, I have quoted that part of the second paragraph and the PvP bonus activation link is below the quote.