Same for me, stuck at 85% because of it. My guess is the validator lands on an edge case that none of the tests do not, e.g. ZZ->AA transition or 999->001. But it’s always tough to debug those, naturally.
*Edited: Solved it by iterating plate by plate. Best hint I can give is consider the edge case where “carrying the 1” trickles all the way from the middle to the right to the left side.