This is only a bug if you value highly visual UI feedback. This issue related to the fact that there are 2 items of visual feedback for tank stance activation, one in the Status Info (Enhancements) bar, and one in the class gauge UI element.

Class/Level: any tank class that has unlocked its gauge UI element and its tank stance skill
Steps:
1. Activate tank stance, e.g. Royal Guard for Gunbreaker
2. Observe "Status Info (Enhancements)" bar elements and tank stance gauge visual element
3. The tank stance shows up in both places, and as such one of those places is redundant visual information and can be removed.

Pics: