10% more duration mastery not working?

In my skill stat sheets my duration modifier is 90%, which is what I would expect if I had not taken the 10% more skill effect duration mastery.

You have 90% increased skill effect duration.

The 10% more skill effect duration from the Mastery is multiplicative with all of your other modifiers to skill effect duration.

For example, if your skill's duration would have been 10 seconds and has no other modifiers, your 90% increased skill effect duration brings it to 19 seconds, and then your 10% more skill effect duration brings that to 20.9 seconds.

Seems strange to me that the 10% more from the mastery does not appear in the "Skill Duration Modifier"
I would expect that if I had 100% increased and 10% more I would see 110% under the Skill Duration Modifier.

It is really hard to actually test to see if I'm getting the effect out of this that I should.

Actually it looks like it is working.
I started a timer at the same time as placing a storm brand and it lasted 16 seconds detached which matches what it says in the duration of the skill tooltip.
without the 10% more the storm brand lasts around 14 seconds and the tooltip reflects this.

So no issue it seems, its just hard to tell the multiplier is taking effect without checking the tooltip and comparing it to the base duration +% increased amounts and +10% more