Found more chatroom specific code in converse-core.js

src/converse-core.js

@@ -2284,9 +2284,7 @@
             onChatBoxAdded: function (item) {
                 var view = this.get(item.get('id'));
                 if (!view) {
-                    if (item.get('chatroom')) {
-                        view = new converse.ChatRoomView({'model': item});
-                    } else if (item.get('box_id') === 'controlbox') {
+                    if (item.get('box_id') === 'controlbox') {
                         view = new converse.ControlBoxView({model: item});
                     } else {
                         view = new converse.ChatBoxView({model: item});

src/converse-muc.js

@@ -123,6 +123,17 @@
             },
 
             ChatBoxes: {
+                onChatBoxAdded: function (item) {
+                    var view = this.get(item.get('id'));
+                    if (!view && item.get('chatroom')) {
+                        view = new converse.ChatRoomView({'model': item});
+                        this.add(item.get('id'), view);
+                        this.trimChats(view);
+                    } else {
+                        this._super.onChatBoxAdded.apply(this, arguments);
+                    }
+                },
+
                 registerMessageHandler: function () {
                     /* Override so that we can register a handler
                      * for chat room invites.