Frenzy charges with Snakebite Assassin's mits?

Is there an issue with the use of the gloves where using the skill "frenzy" at max frenzy charges will not refresh the frenzy timer?

When I use the snake bite assassin's mitts I am unable to consistently keep max frenzy charges as when I reach 8 (my max) the duration of frenzy is about 4 seconds (from increased duration), however each subsequent frenzy skill does not refresh the 4 second timer, instead it decreases to 0 where then I will need to build the charges back up.