As in todays patchnotes is stated:
"Prime: Shadow Brand- Added the Assassin's Mark effect to the following skills: Prime: Fox Claw (I to III), *Prime: Shackles*"
But it doesn't provide any brand.
It seems like an display error. The monsters indeed becoming weaker, while fox claw or other skills are indeed create a brand.
May try repair mode. Hope that fixes it.