skill duration mastery - points in 10% more and 10% less duration show no effect

Hello,

I put one skill point in the duration mastery "10% more skill effect duration, under potency of will.
After that it does not show any percentage increase in the main menue under MISC, skill duration modifier.
The percentage number stays the same.

I also tried the mastery point "10% "less skill effect duration". This had also no effect on my skill duration modifier under MISC in the menue. The number should have decreased, but stayed the same.

Is it my fault, did I miss something, or is it simply not working ?

kind regards

Arkon