[1.21] Mixins get ClassNotFoundException when referencing the mod's Kotlin classes

[filloax]

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Hi, I'm using KFF 5.6.0 in my 1.21 neoforge mod, but when it loads mixins I get this error when the mixin code tries to use a Kotlin class (even just a static reference):

Exception in thread "main" java.lang.RuntimeException: org.spongepowered.asm.mixin.transformer.throwables.MixinTransformerError: An unexpected critical error was encountered
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:32)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:53)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:71)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.Launcher.run(Launcher.java:103)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.Launcher.main(Launcher.java:74)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.BootstrapLaunchConsumer.accept(BootstrapLaunchConsumer.java:26)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.BootstrapLaunchConsumer.accept(BootstrapLaunchConsumer.java:23)
	at [email protected]/cpw.mods.bootstraplauncher.BootstrapLauncher.run(BootstrapLauncher.java:210)
	at [email protected]/cpw.mods.bootstraplauncher.BootstrapLauncher.main(BootstrapLauncher.java:69)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at [email protected]/net.neoforged.devlaunch.Main.main(Main.java:55)
Caused by: org.spongepowered.asm.mixin.transformer.throwables.MixinTransformerError: An unexpected critical error was encountered
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:392)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:250)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.service.modlauncher.MixinTransformationHandler.processClass(MixinTransformationHandler.java:131)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.launch.MixinLaunchPluginLegacy.processClass(MixinLaunchPluginLegacy.java:131)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.serviceapi.ILaunchPluginService.processClassWithFlags(ILaunchPluginService.java:156)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.LaunchPluginHandler.offerClassNodeToPlugins(LaunchPluginHandler.java:94)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.ClassTransformer.transform(ClassTransformer.java:120)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.TransformingClassLoader.maybeTransformClassBytes(TransformingClassLoader.java:57)
	at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.readerToClass(ModuleClassLoader.java:190)
	at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.lambda$findClass$20(ModuleClassLoader.java:298)
	at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.loadFromModule(ModuleClassLoader.java:311)
	at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.findClass(ModuleClassLoader.java:298)
	at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.loadClass(ModuleClassLoader.java:214)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
	at java.base/java.lang.Class.getDeclaredMethods0(Native Method)
	at java.base/java.lang.Class.privateGetDeclaredMethods(Class.java:3578)
	at java.base/java.lang.Class.privateGetPublicMethods(Class.java:3603)
	at java.base/java.lang.Class.getMethods(Class.java:2185)
	at MC-BOOTSTRAP/[email protected]/net.neoforged.fml.earlydisplay.DisplayWindow.updateModuleReads(DisplayWindow.java:605)
	at MC-BOOTSTRAP/[email protected]/net.neoforged.fml.loading.ImmediateWindowHandler.acceptGameLayer(ImmediateWindowHandler.java:94)
	at MC-BOOTSTRAP/[email protected]/net.neoforged.fml.loading.FMLLoader.beforeStart(FMLLoader.java:160)
	at MC-BOOTSTRAP/[email protected]/net.neoforged.fml.loading.targets.CommonLaunchHandler.launchService(CommonLaunchHandler.java:115)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:30)
	... 11 more
Caused by: org.spongepowered.asm.mixin.transformer.throwables.MixinPreProcessorException: Attach error for growsseth.mixins.json:client.RuinsMapIcon_GuiGraphicsMixin from mod growsseth during activity: [Transform -> Method onRenderItemDecorations(Lnet/minecraft/client/gui/Font;Lnet/minecraft/world/item/ItemStack;IILjava/lang/String;Lorg/spongepowered/asm/mixin/injection/callback/CallbackInfo;)V -> GETSTATIC -> com/ruslan/growsseth/item/GrowssethItems::RUINS_MAP:Lnet/minecraft/world/item/MapItem;]
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.attach(MixinPreProcessorStandard.java:313)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.createContextFor(MixinPreProcessorStandard.java:277)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinInfo.createContextFor(MixinInfo.java:1289)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.apply(MixinApplicatorStandard.java:187)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.TargetClassContext.apply(TargetClassContext.java:437)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.TargetClassContext.applyMixins(TargetClassContext.java:418)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:363)
	... 33 more
Caused by: java.lang.RuntimeException: java.lang.ClassNotFoundException: com.ruslan.growsseth.item.GrowssethItems
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.transformMemberReference(MixinPreProcessorStandard.java:791)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.transformField(MixinPreProcessorStandard.java:782)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.transform(MixinPreProcessorStandard.java:745)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.attach(MixinPreProcessorStandard.java:307)
	... 39 more
Caused by: java.lang.ClassNotFoundException: com.ruslan.growsseth.item.GrowssethItems

This doesn't happen in the Fabric version of the mod, where the mixins can use the Kt classes with no issue - am I missing something, or is this a bug?