Fragment of destruction cool down bug?

Hardcore DH, GoD (LesSoul character). My Vengeance cooldown is just about 20 seconds. With no other skills on cooldown, Fragment of Destruction with the 1 second Mark of Destruction CDR - I see no cooldown ticks when running through low level trash.

UPDATE: I was running too low of levels and killing trash before actually passing through them. On higher GR it works fine.