On the latest update, 3.0.17, having the Cowardice enchantment on any equipped armor piece halves the player's max HP.

Fixed for next update

Problem: Cowardice enchantment removes 60% of maximum health when on equipped armor.

Forge 36.2.0
Minecraft 1.16.5
MCP 20210115.111550
4 mods loaded: Minecraft, Forge, Dungeons Gear, Spice of Life: Carrot Edition

Fixed for next update

dungeons_gear-1.16.5-3.0.17.

Steps to replicate:

1. Create a new creative world with default settings.
2. Apply Cowardice to leather pants
3. Go into survival mode
4. Equip the leather pants

I suspected it was Spice of Life: Carrot Edition originally, but I tested with and without it installed and at various maximum HP values. Results do not differ.

Tested it against a regular zombie, using the Spear from this mod. No other modifiers and no critical hits, all attacks are made after the attack indicator has filled all the way up. Without Cowardice III a hit does 3 hearts, with it while at full health does 7.5 hearts and while not at full health does 5.5 hearts. Looks to me like both Cowardice and Reckless are present

Can confirm this happened to me as well. I think it got mixed up with the Reckless enchantment since I lost 6 hearts (60% of my max health)