Did you restart your game?
How are you editing the config file in-game? As far as I know, Forge removed that feature.

Did you spawn new mobs after changing the config? It doesn’t work on old ones even if you reload.

yes, i create a new world and spawn new mobs after changing the config, it still keep the default health value.

Did you restart your game? How are you editing the config file in-game? As far as I know, Forge removed that feature.

Yes, I also tried to modify the config file directly, but it still not working.
By using two mods, "Configured" and "Game Menu Mod Options".
(btw. I have tried to modify the older version 1.5.19 and there is no problem)

can confirm, tried to give ferrous wroughtnaut a 3x health and 2x damage mod, neither applied.