I'm setting players health with attributes and I've been using attributes to set the max health:

    @SubscribeEvent
    public void setMaxHealthOnRespawn(PlayerEvent.PlayerRespawnEvent event) {
        IAttributeInstance health = event.player.getEntityAttribute(SharedMonsterAttributes.MAX_HEALTH);
        health.setBaseValue(4);
    }

But I'm having problems with ToughAsNails mod. I tried this without ToughAsNails (by building my mod and not installing ToughAsNails because its optional for my mod) it worked fine I was at 2 hearts when I respawned but when I install ToughAsNails I get stuck in reboot loop without any errors and it only printed one killed by message in chat.

Github

It's usually a bad idea to set the base value of health. Is there a reason you can't do this with multiplier attribute modifier?

If you can't to it, setting the event priority to LOWEST might work. It marks the event subscribing method to get called on the last pass.