“Every 3 casts of Bestial Wrath, you surrender to darkness for 12 sec.”
If you have 1 or 2 stacks of Building up to a Withering Fire buff and the boss fight starts/ends, your stacks reset. Only occurs in raids and dungeons (except timewalking).
Reported as a bug, got a suggestion to reset my UI - still don’t know how these two are related but tried it and the buff still resets.