package me.deftware.mixin.mixins.item;

import me.deftware.client.framework.registry.ItemRegistry;
import net.minecraft.item.Item;
import net.minecraft.item.Items;
import net.minecraft.util.Identifier;
import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.injection.At;
import org.spongepowered.asm.mixin.injection.Inject;
import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;

@Mixin(Items.class)
public class MixinItems {

	@Inject(method = "register(Lnet/minecraft/util/Identifier;Lnet/minecraft/item/Item;)Lnet/minecraft/item/Item;", at = @At("TAIL"))
	private static void register(Identifier id, Item item, CallbackInfoReturnable<Item> ci) {
		ItemRegistry.INSTANCE.register(id.getPath(), item);
	}

}

package me.deftware.mixin.mixins.item;

import me.deftware.client.framework.registry.ItemRegistry;
import net.minecraft.item.Item;
import net.minecraft.item.Items;
import net.minecraft.util.Identifier;
import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.injection.At;
import org.spongepowered.asm.mixin.injection.Inject;
import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;

@Mixin(Items.class)
public class MixinItems {

	@Inject(method = "register(Lnet/minecraft/util/Identifier;Lnet/minecraft/item/Item;)Lnet/minecraft/item/Item;", at = @At("TAIL"))
	private static void register(Identifier id, Item item, CallbackInfoReturnable<Item> ci) {
		ItemRegistry.INSTANCE.register(id.toString(), item);
	}

}

--- 
+++ 
@@ -14,7 +14,7 @@
 
 	@Inject(method = "register(Lnet/minecraft/util/Identifier;Lnet/minecraft/item/Item;)Lnet/minecraft/item/Item;", at = @At("TAIL"))
 	private static void register(Identifier id, Item item, CallbackInfoReturnable<Item> ci) {
-		ItemRegistry.INSTANCE.register(id.toString(), item);
+		ItemRegistry.INSTANCE.register(id.getPath(), item);
 	}
 
 }
Use paths instead of toString
