I am playing a swordsman(Corsair3) at the moment and noticed a pair of issues with the Concentrate skill.
One, which I havent found how to reproduce yet but is happening often is Concentrate reverting to lvl1 or lower than maximum level at least. (And appears on skill screen as such). Having to relog or change channel to be able to keep using it at its actual level.
The other being Concentrate not applying on Guns C(offhand) attacks. Which was easily tested as damage did not change with or without the buff.