The only things I've found to degrade the coating is either TB cleaner used without removing the TB from the SC or the meth injection. Honestly..the amount you have missing.....my money is on the injection. TB cleaner is usually a much lesser degree.
I have never used any TB cleaner, or anything else for that matter.
One interesting observation when inspecting the coatingless rotor is some scuffing on the snout end of the rotor in one location about 1 square inch in size.
I suspect two possibilities. As McGrath stated, it could be velocity/pressure related.
Or it could be the sudden cooling of the injection effect on the hot rotor. Expansion/contraction hence coating delamination.
If you look at the edges of the coating, they are not smooth like erosion but look as if the coating simply broke away.