Hi Sorrell
TB, with all its horrible implications, is fortunately rare, so please don't resign yourself to that conclusion while there is still any hope that it could be something else.
Bacterial infections can be pretty awful themselves, and are common among bettas. Could it be possible that you have a mutated strain of some ordinary bacteria in your tanks? This can happen fairly easily, from what I have heard. By using any medication, (especially antibiotics) and not treating long enough to completely cure the fish, there is a risk that when the disease comes back it will be in a form that is resistant to that medicine. I know I've seen this happen with my own.
Remember too, that this might or might not have happened in the course of any treatment you gave them. When we buy new fish we never know what nasties they might be bringing with them. And it's possible too that a fish might be ill for a time before any symptoms become evident. In this way it would be fairly easy to cross contaminate a number of tanks before realizing that there is a problem.
Another thing too, is that unless a fish has become stressed in some way, the bacteria in question could remain in the water for a long time before it shows itself.
Does any of this seem possible in your case?