There are several options for sorting the lists. It is easy to change it, and the effects are immediate. However, the drag-and-drop sort is a specific one, and it deserves special treatment when changing sorting to another from it.

The example will be useful to show the problem. If you have your drag-and-drop tasks order {C, A, D, B} and you select "sort by name", you will get {A, B, C, D}. But if you want to get back to drag-and-drop sorting, you will never see {C, A, D, B} again. You will have a drag-and-drop sorting, but the order will still be {A, B, C, D}.
Not a big deal UNLESS you accidentally change the sorting option on the list where you have painstakingly arranged dozens (or hundreds) of tasks just by confusing it with another list. In the blink of an eye, you lose all your work, with no way to come back.

That is why I propose to display a warning when you try to change the drag-and-drop sort to any other. It should say that "You are on the list of a of manually arranged tasks, changing the sort method will irrevocably change the order. Are you really sure to do that?"

The other option will be to provide a mechanism to lock the sorting method for specific lists. In this way, you will instantly see that it is probably not the list you want to change. And until you unlock it, changes will not be possible at all. This will also cause a moment of reflection before making an irreversible change, but it is probably harder to implement.