Negative damage is a result of overflowing the memory of the damage variable, so as a fix to this the developer could either change all the negative damage dealt to a maximum possible damage the variable can handle or increase the allocated memory for it. That being said I haven't the experience with terraria mods so I don't know if the developer of the mod can access it. Another way to fix it could be dividing all the damage and health modifiers by a 100 for instance, which would also help with balancing the mod.

Yeah I have this problem too. Doing a melee build and I've gotten to the point where I'm level 2371. I'm playing with Calamity using the Murasama and I've reached a point in damage where the sword doesn't even show up when I try to hit an enemy. Although, I know something is still getting registered because I'm getting lifesteal for like 3mil in green text above my player. I wish there were a fix for this.