Fix

versions/1.21.4/src/main/java/org/saturnclient/impl/mixins/SpriteMixin.java

@@ -0,0 +1,9 @@
+package org.saturnclient.impl.mixins;
+
+import net.minecraft.client.texture.Sprite;
+import org.saturnclient.common.ref.asset.SpriteRef;
+import org.spongepowered.asm.mixin.Mixin;
+
+@Mixin(Sprite.class)
+public abstract class SpriteMixin implements SpriteRef {
+}

versions/1.21.4/src/main/resources/impl.mixins.json

@@ -10,7 +10,8 @@
     "MinecraftClientMixin",
     "QuaternionfMixin",
     "WindowMixin",
-    "TextMixin"
+    "TextMixin",
+    "SpriteMixin"
   ],
   "injectors": {
     "defaultRequire": 1