That is not at all how it works. It reduces 20% to stamina. There base health and stamina are not the same thing.

Yenney:

Actually [if I'm looking at this correctly, and If I'm not, forgive me] stamina and health are directly proportional. Each point of stamina provides 10 health points. So detracting 20% of stamina would in fact remove 20% health.

Say I have 27,000 health. That means I have 2,700 stamina points. Removing 20% of that stamina means removing 540 stamina. The end result is a total of 2,160 stamina. This equates to 21,600 health points. Now, if you just subtract 20% of the original health to begin with, you subtract 5.400 health, and arrive at the same number of health points: 21,600.

freeway, not all of somethings HP comes from stamina. Some things have base health, which means even if the target has 0 stamina, it'll have a certain amount of HP.

Most elite monsters have a high amount of base health and not as much stamina, to prevent stamina debuffs from toasting them.

Anyways, yeah this sucks now. The whole point was that stamina loss makes the target easier to kill, without the stamina loss its pretty much garbage.