Balm for Minecraft 1.21.4 has introduced a load context for Forge.
ForgeLoadContext on ForgeYou must change your Forge entrypoint to pass in a ForgeLoadContext with the mod event bus from your FMLJavaModLoadingContext.
public ForgeWaystones(FMLJavaModLoadingContext context) {
final var loadContext = new ForgeLoadContext(context.getModEventBus());
Balm.initializeMod(Waystones.MOD_ID, loadContext, Waystones::initialize);
}