Raid 2 bosses' immunity to Knockback is preventing the secondary Damage effect from Rune Magus Rem and Kiera.
Both skill descriptions read "If affected unit(s) cannot be knocked back, deal damage equal to XX% of hero's MAG"
The boss should not fully "Resist" knockback like in PVP but rather take damage as raid bosses positions are fixed.
An indirect buff to the underused Rem & Kiera will also be widely welcomed by the community.