There's a lot to say here because there are a few apparent misunderstandings but I guess we'll start with the big thing:
The only way to fix this, is then to somehow find those members and give them their original title name via Custom Title in their profle. Hours of work!
That is not at all what you would need to do. You're conflating two different things here. We have a user title ladder, which is what your bug report is about, and we have custom titles.
The user title ladder is a list of available user titles that users will dynamically switch between depending on the criteria you set. Custom titles are a totally static value that can be applied per member. There are also user group titles which can override the user title ladder.
They appear in the same place on a user's profile but they're all separate. If either the user title ladder value or group title becomes unset, the solution certainly isn't to apply custom titles manually. Custom titles are for one-off or exceptional values that are designed to be manually set if that is required. To give multiple users the same title, setting custom titles is not the way.
I think that if you delete a Ttitle Name from the Ladder, then the system should leave the names of members as they were.
Well, no. It is essentially a lookup table of values, almost like a ranking system. You can add or remove or change values from the user title ladder as much as you like. That's what it's for. If for some reason the values stuck around after they were deleted then how would you ever replace or rename a title you didn't want anymore? Generally if you delete something, by design, it is supposed to be deleted.
In this case if you have "Old member" with a value of 100 messages and "Not so old member" with a value of 50 messages and "New member" with a value of 0 messages, then obviously if the "Not so old member" one is deleted the correct behaviour is anyone who had between 0-100 messages would now be "New member" because that is now the correct value to have based on the values provided in the user title ladder.
Or revert to the next level down! Not New Member
That is exactly what happens already and is consistent with my example above. If your users got "New Member" then that's the correct value for them after you deleted the previous value.
The scheduled task would have taken care of it, or you could have run it manually.
Don't do this. Run the user title task.
Sorry but this is totally misleading. It's just not how the user title ladder is managed or built. There is no "scheduled task" or task to run.
The correct solution is simple: You just need to add the previous value back in to the user title ladder screen:
In the field labelled "New" you just need to write the previous title back in and set the desired minimum value and click "Update user titles". If you put in exactly the same values as there were before you deleted them then it would apply instantly and automatically to all members.
