I don't think much is really known about this to tell you the truth. There is such a huge spectrum of root canal failures based on who did the root canal, how they did it, materials used and if there were files left or an overfill issue. From what I read slight overfill the body can adjust or you can go in from the top of the root and remove the material to remove the pain. But it varies from procedures done by a dentist (who might be less experienced in the procedure) vs and Endo who is trained specifically for root canals.

My theory is at times the body rejects the tooth just because it is dead now but that's just my thoughts. I am very sorry your in pain we are in the same time line of having our teeth worked on. It's terrible. I am considering getting on an antidepressant to see if it helps the pain. Pulling is also an option I am considering and may end up doing that if I don't get an answer. I might seek a ENT or facial pain specialist at this point. Wishing you nothing but the best.