base 1.20.1 instance
mods installed are fabric api & eeac
(screenshots shown use the mod jade for easier viewing of health)

all mobs that have a health value higher than 20 hit-points have been defaulted to 20 (including boss mobs such as the wither and ender dragon). if i had to guess this is likely caused by the addition of overshield in 1.6, as this bug isn't present when using version 1.6-b1

(1.6)

(1.6-b1)

(1.6)

(1.6-b1)

(1.6)

(1.6-b1)

Overshield is 100% the issue. I have a potential fix in mind but I won't be able to test it and implement it until next wendsday at least

I just managed to fix it. The new version (1.6.1) is available on modrinth