I have reported this bug multiple times since ptr via the ingame options, but that doesn’t ever seem to reach anyone, so Ill try my luck here. Also, this topic has been brought up before, well over a year ago (allthough in US forums: https://us.forums.blizzard.com/en/wow/t/demon-hunter-soul-furnace-talent-bugged/1409916)
To simply say it with Marmota’s words:
"When you absorb a soul fragment you get a stacking buff called “Soul Furnace”
When you get to 10 stacks of that buff you get the damage buff called “Soul Furnace”
Both buffs have the exact same name.
The damage buff Soul Furnace gets over written by the stacking buff Soul Furnace and when that happens it goes away. So, when you get more than 10 stacks you only have a couple seconds between the time you get the damage buff and you absorb another soul fragment to use the damage buff."
I felt he explained that very well, and this still remains the very same issue to this day. Only, with the new tier 4 set it practically becomes unplayable, since you no longer have direct controll over your soul fragment generation. In practice you are loosing 9/10 empowered casts because the buff gets immediately overwritten by itself in most cases.
PLEASE fix this, Blizzard.