Dark Knight Limit Break - Dark Force BUG?

Okay so I was playing Alex(Burden) today. My party was nearly dead and the boss was casting discold for an orb phase, I used the LB lvl 3 to save my party through the phase. When the LB buff was applied, I realized that it was only an 8 secs buff. Now according to consolegameswiki, ffxivinfo and wikia, tank limit break level 3 duration is 12 secs. This made me confused so I reviewed my video.(I had the fight recorded) I used stopwatch and when I clicked the LB button in the video I started counting until LB buff wore off. All that was still roughly 10 secs. I've also checked out some footages of other tanks LB lvl 3 and I see the same 8 secs buff. Is this a bug? or is this normal?