I would tend agree with you that two cracks in the same place is no fluke. The most of the heat coming off the dome travels up the interior side of the flue opeing (ask the burned hair on my arm how I know
). Combine that with that side also being in contact or close proximity to the dome and you have a very high temp gradient across it. That seems the most likely explanation.
So then, what to do? I think you have to either abondon using the tile in that spot or replace it with something else. If you still want to use the tile you could try replacing the verticle transition with something built out of refractory bricks or KS-4. That may give you enough mass to help balance the temps out, especially if it is at least 14" or so tall. You could also try using mortar to lay a few verticle lines or beads in a slightly twisted line on the inside walls of your vent. These may help induce a turbulence into the flue to help evenly distibrute or mix the heated air.
I wish I could give better answers, but I used KS-4 and duravent so no direct experience.