I wanted to make the bosses a fair bit harder since bosses got VERY easy once we got into the beginning of the mid game stage. All I did was change S:"Max Health Multiplier"=6.0 to S:"Max Health Multiplier"=18.0 and the min health to 6 as well as adjust a couple spawner variables to make cave spiders a little less frequent. But now all "BOSS" mobs just have the normal 20HP even after reverting changes.

I'm not experiencing that behavior
Bosses are retaining their hp modifiers just fine, this one was generated with a range of 9x - 18x

Interestingly enough, upon further inspection, boss hp is being cut in half for some reason after generation. It's fine when they're added to the world, but once generation finishes and the entity is recreated, it loses half of it's health.

Well glad I could help find that bug. also I am on 1.16.1 version of the mod because I am playing the All The Mods 6 Pack.

Bosses are generated at worldgen, so be sure you're only looking in new chunks after changing the values.
Also, you may need AttributeFix to use higher multiplier values since attributes break above a certain threshold

I already have attribute fix and yes I was looking in new chunks using the end bug to easily spot the bosses.

Upon even further inspection, what I'm seeing is a hwyla bug on entity hp values being displayed: TehNut-Mods/HWYLA#298
So this isn't a real issue, it seems.