Specify the mappings dependency only in the root project, and don't shadow every dependency.

build.gradle.kts

@@ -15,14 +15,22 @@ architectury {
 }
 
 subprojects {
+    lateinit var MappingsDep: Dependency
     apply(plugin = "org.jetbrains.kotlin.jvm")
     apply(plugin = "forgified-fabric-loom")
     apply(plugin = "com.github.johnrengelman.shadow")
     val shadowC by configurations.creating
     tasks.withType(ShadowJar::class) { this.configurations = listOf(shadowC) }
+    extensions.configure<net.fabricmc.loom.LoomGradleExtension>("loom") {
+        MappingsDep = officialMojangMappings()
+        silentMojangMappingsLicense()
+        mixinConfig("snowygui-mixins.json")
+        refmapName = "snowygui-common-refmap.json"
+    }
     dependencies {
         shadowC(kotlin("stdlib-jdk8"))
         "minecraft"("com.mojang:minecraft:${rootProject.property("minecraft")}")
+        "mappings"(MappingsDep)
     }
 }
 

common/build.gradle.kts

@@ -9,12 +9,5 @@ repositories {
 }
 dependencies {
     compileOnly("org.spongepowered:mixin:0.8-SNAPSHOT")
-    mappings(loom.officialMojangMappings())
-}
-
-loom {
-    silentMojangMappingsLicense()
-    mixinConfig("snowygui-mixins.json")
-    refmapName = "snowygui-common-refmap.json"
 }
 

fabric/build.gradle.kts

@@ -13,7 +13,6 @@ object Groups {
 architectury { platformSetupLoomIde(); fabric() }
 
 dependencies {
-    mappings(loom.officialMojangMappings())
     modApi("${Groups.Fabric}:fabric-loader:${rootProject.property("fabric_loader_version")}")
     modApi("${Groups.FabricApi}:fabric-resource-loader-v0:${rootProject.property("fabric_resource_loader_version")}")
     include("${Groups.FabricApi}:fabric-resource-loader-v0:${rootProject.property("fabric_resource_loader_version")}") { isTransitive = false }
@@ -23,5 +22,5 @@ dependencies {
     implementation(project(path = ":common")) { isTransitive = false }
     add("developmentFabric", project(path = ":common")) { isTransitive = false }
     shadowC(project(path = ":common", configuration = "transformProductionFabric")) { isTransitive = false }
-}; loom { silentMojangMappingsLicense() }
+}
 

forge/build.gradle.kts

@@ -1,8 +1,7 @@
 architectury { platformSetupLoomIde(); forge() }
 dependencies {
-    mappings(loom.officialMojangMappings())
     forge("net.minecraftforge:forge:${rootProject.architectury.minecraft}-${rootProject.property("forge_version")}")
     implementation(project(path = ":common")) { isTransitive = false }
     add("developmentForge", project(path = ":common")) { isTransitive = false }
     shadowC(project(path = ":common", configuration = "transformProductionForge")) { isTransitive = false }
-}; loom { silentMojangMappingsLicense(); useFabricMixin = true }
+}; loom { useFabricMixin = true }