Suspect this is the fix to #203, but I'm unsure

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/TransitionSessionHandler.java

@@ -19,6 +19,7 @@ import com.velocitypowered.proxy.protocol.packet.Disconnect;
 import com.velocitypowered.proxy.protocol.packet.JoinGame;
 import com.velocitypowered.proxy.protocol.packet.KeepAlive;
 import com.velocitypowered.proxy.protocol.packet.PluginMessage;
+import com.velocitypowered.proxy.protocol.util.PluginMessageUtil;
 import java.io.IOException;
 import java.util.Collection;
 import java.util.concurrent.CompletableFuture;
@@ -134,6 +135,14 @@ public class TransitionSessionHandler implements MinecraftSessionHandler {
       return true;
     }
 
+    // We need to specially handle REGISTER and UNREGISTER packets. Later on, we'll write them to
+    // the client.
+    if (PluginMessageUtil.isRegister(packet)) {
+      serverConn.getPlayer().getKnownChannels().addAll(PluginMessageUtil.getChannels(packet));
+    } else if (PluginMessageUtil.isUnregister(packet)) {
+      serverConn.getPlayer().getKnownChannels().removeAll(PluginMessageUtil.getChannels(packet));
+    }
+
     // We always need to handle plugin messages, for Forge compatibility.
     if (serverConn.getPhase().handle(serverConn, serverConn.getPlayer(), packet)) {
       // Handled, but check the server connection phase.

proxy/src/test/java/com/velocitypowered/proxy/util/collect/CappedCollectionTest.java

@@ -44,6 +44,9 @@ class CappedCollectionTest {
     assertTrue(coll.addAll(doesFill2), "did not add items");
     assertThrows(IllegalStateException.class, () -> coll.addAll(overfill),
         "items added to collection although it is too full");
+
+    assertFalse(coll.addAll(doesFill1), "added items?!?");
+
     assertEquals(3, coll.size(), "collection grew in size unexpectedly");
   }
 }