This test calls setFCV on a replica set, which inserts the FCV document with majority write concern, then tries to downgrade its binaries, which starts with the set's secondaries. If one of the secondaries hasn't replicated the FCV document before it is put into maintenance mode as part of downgrade here, it will hit this fassert when apply the batch with the FCV document.

The test should wait for replication before downgrading to avoid this problem. This test only exists on the 3.6 branch.