There are two similar ‘lose health over time’ effects on the lich:
A) Health Drain such as is found on the nodes ‘Blood Pact’ in the Acolyte passive tree and ‘Unclosing Wounds’ in the Lich passive tree.
B) Decay which is used by the Reaper form to act ‘as a timer’.
It appears that ‘Decay’ is not lowered or prevented by effects that reduce ‘Health Drain’ such as the Volatile Blood node in the Lich tree. This is likely intended such that you can’t stack these effects (or use Death Seal) to get infinite duration Lich. The issue is that the Skill node ‘Mistress of Decay’ found in the Reaper Form passive tree says that “Your health DECAYS more slowly” and then only gives Drain resistance.
So the ‘Mistress of Decay’ skill node is equating Health Drain and Health Decay. Either that helptext needs to be changed, additional effect needs to be listed in that node for the actual Decay reduction (should that actually happen I didn’t bother testing), or the Reaper form health Decay should be effected by Drain resistance.