AFAIK, the "Delete Group" option is available for the Custom Groups that the user creates using the My Groups feature and never the default program title groups.
OK, on my 722K with SW L667, from the My Recordings list, if I select one of my Custom Groups, the Delete Group button is available and does function. However, if I enter any of the NON-custom groups such as the Group by Title default (you did say that you have them grouped by title as this will still occur even if you activate your custom groups, so no need or point to create a custom group according to titles of recordings as, again, the DVR will do this automatically by selecting Group by Title or My Groups), the Delete Group button is grayed out. I believe this has always been the case, and, therefore, is operating properly..
Are you saying that Delete Group button was always available from within the default Group by Title? I may have missed that, and you could be correct. However, one of the reasons the Delete Group option is available from within a Custom Group and not the default Group by Title is that there can be sub-groups within the Custom Group, specifically, the Group by Title default, among others--again, within the Custom Group. In this situation, a DELETE GROUP button has a purpose, but as explained below, not really needed within the default Group by Title.
There is a way for you to issue a global delete of your default Group by Title recordings without having to select each one:
1.First, select the default Group by Title group (this step must ALWAYS be performed first or you risk deleting all your recordings--every last one on the entire DVR--in the entire My Recordings list.)
2.Then select the EDIT button.
3.Now select the SELECT ALL button. This selects all the recordings within this group only.
4.Now select, DELETE, and all the recordings in that group will be deleted and the group itself is deleted as a consequence because there are no longer any of those titles on the list, therefore, no group.
I believe this is how it all worked from day 1, but I may be mistaken. Good luck.