broadcastChanges was called on client (even in vanilla code) all this time and found out only now :)

src/main/kotlin/ru/dbotthepony/mc/otm/menu/MatteryMenu.kt

@@ -353,14 +353,20 @@ abstract class MatteryMenu(
 	}
 
 	override fun broadcastChanges() {
-		super.broadcastChanges()
-		matteryBroadcast()
+		// how did mojang missed this
+		if (player is ServerPlayer) {
+			super.broadcastChanges()
+			matteryBroadcast()
+		}
 	}
 
 	override fun broadcastFullState() {
-		synchronizerRemote.invalidate()
-		super.broadcastFullState()
-		matteryBroadcast()
+		// how did mojang missed this
+		if (player is ServerPlayer) {
+			synchronizerRemote.invalidate()
+			super.broadcastFullState()
+			matteryBroadcast()
+		}
 	}
 
 	override fun stillValid(player: Player): Boolean {