Send signature headers when chat event viewers are modified

Fixes #8275

patches/server/Adventure.patch

@@ -547,6 +547,9 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +                    viewer.sendMessage(source, audienceMsgFunction.apply(viewer), MessageType.CHAT);
 +                }
 +            }
++
++            // Make sure to send remaining headers
++            ChatProcessor.this.outgoing.sendHeadersToRemainingPlayers(ChatProcessor.this.server.getPlayerList());
 +        }
 +
 +        private void sendToViewer(final CommandSender viewer, final ChatType.Bound chatType, final @Nullable Function<Audience, net.minecraft.network.chat.Component> msgFunction) {