When the Attribute generic.max_health is used to raise the player's health above max then the player saves or logs out the players health isn't saved. Instead it is capped at ten hearts. I found this plugin which fixes this. But I wish there were a forge mod for this.

Thanks for the suggestion. I will look into this. Which MC version are you using?

1.16.5

I've made a new mod to fix this specific issue. You can find it here. https://www.curseforge.com/minecraft/mc-mods/max-health-fix