Just tested on gauntlet and both the duration are the effect are working fine for me
65k health reduction
225k HP Matador
160k Matador with debuff
Edit: It’s just the way increased/reduced health mechanics work. They increase/reduce current HP so the %HP stays the same.
If an enemy has 50% of basic HP before being marked, they will still have 50% of modified HP after mark .
This happens with Heimlock stirilize or Fortress bronze. Try using any of them with 1%HP, you will barely see any increase.