Implemented Placeholders for onattack mechanic are not working

The Implemented Placholers for onattack (<skill.var.damage-type>, <skill.var.damage-amount>,<skill.var.damage-cause>) are not working and returning "Undefined"

Soul_Drain:
  Skills:
  - onattack{onattackskill=Soul_Drain_Attack;charges=1;ticks=220} @self
  - sound{sound=block.fire.extinguish} @self
Soul_Drain_Attack:
  Skills:
  - message{message="&aamount>> &f<skill.var.damage-amount>"} @self
  - message{message="&atype>> &f<skill.var.damage-type>"} @self
  - message{message="&acause>> &f<skill.var.damage-cause>"} @self

Additional Info: Server Version: 1.21.4 Paper Mythicmobs Version: 5.10.1 (dev build)