Tema is the only one programming this, thank you very much, and it is most likely that either the bug hasn't been reported to him or just flat out isn't fixable, in the meantime, kill mobs hoping to get the thing, if you're extremely lucky you'll get one
Also this is only fixable by @Administrator and above, They would have to access your player file to revert your hearts to it's original state. As well as we would have to take any and all Souls you have made/used in creation of items. However that is up to Admins if they want to go forwards with this.
You don't need to do anything for the time being, just wait for an @Administrator to edit your nbt data for you. Actually on any server but horizons we Sr.mods can fix this issue using /nbt *<name of online player> Attributes[0].Modifiers[0].Amount = -0.1 which will set the number of missing hearts to 1, or if the player is offline then use the same command but replace the * at the start of the username with a @ instead.. though for some reason the powernbt plugin is currently broken on horizons otherwise I would have sorted this out myself already
