The mod seems to be hard-capped at 6 hearts per limb, equating to about 60 basegame HP with the default settings.

Specifically, when in a 1.20.1 instance with only first aid using the most recent version of everything (I made the instance literally 1 hour ago as of making this issue) with the default configs using the /attribute command to set different health values

this is 20hp

this is 30hp

this is 40hp

this is 50hp

this is 60hp

this is 500,000hp

As far as I can tell this is not a rendering issue either, as when I tested in my main modpack a single splash potion of harming did deal noticable damage to me even when I should have had thousands of HP per limb. I also can't find any obvious way of changing this in the configs, because while they do have various 'max health per limb' settings, those are about the base max health from what I can tell, not the maximum the mod will allow when the health is changed via other means.

When you get to insane values like 500,000 I don't think there would be any easy way of even rendering that much health in the health-screen, but depending on the pack only 6 hearts per limb as a hard maximum could pose problems. (particularly in packs where the mobs have been amped up to 11) I think the best bet would proably be allowing upto 3 rows of hearts per limb (maybe only 2, three might be pushing it) and if the player has more than that use a partially numeric display

(in this example the limb itself has full health at 600,000hp, with the hearts either being able to represent only the first 6 hearts worth of it (meaning for the majority of damage they'll stay at full) OR you could use them like a health-bar where they represent the relative amount of health remaining. Neither is ideal but both would function)

You can change this in the config, check the server config by going to your instance folder -> saves -> find the world you were in -> server config -> find the first aid server config and change the parameter there
By default first aid will always cap all the limbs at 6 hearts (for some reason idk) but you can disable that cap

In other cases for some reason first aid tends to reset the health back to 20 when the player relogs this is not a big issue as it seems to be just visual, like your health is actually there it’s just the UI that displays otherwise, you can try this by having a chestplate or wtv that increases your max health and leaving and joint the world gain see how your health is back at 20