Using the new set and running with bone armour, the duration is very jumpy. I am using Scythe of the cycle which reduces the duration by 4 seconds, however sometimes bone armour will not even outlast its cool down, occasionally resetting all together when engaging a mob 
the duration times seem random and I can not figure out what be causing it to go from say 10 secs to 1 sec
https://us.diablo3.com/en/item/scythe-of-the-cycle-P61_Unique_Scythe1H_03
The Scythe reduces 4 seconds every time you cast secondary skills. ie -8s if you cast two times, -12s if you cast 3 times, and so on.
It is intended. Not a bug.
Well butter my biscuit, I never read it as that before or really even noticed it when using the mages build in season 20, still I find it to be very inconsistent as in when not using a secondary it will jump to expired on its own, or at times not reduce at all.