If a big index key (>1KB) has been inserted in FCV 4.2 and then a downgrade happens, the big index key remains on disk. This makes the index validation fail in FCV 4.0 because FCV 4.0 does not allow big index keys. This behavior is expected as we wrote in the design of PM-284:
It is expected that a big index key inserted in FCV 4.2 causes the index validation in FCV 4.0 fail. The solution is to drop the index and run validate again.
Therefore, the fuzzer should keep in mind this assumption and not validate in this case.