The comment says that this should never happen, so I would suggest that we make it an invariant: https://github.com/mongodb/mongo/blob/e09ce369e4912a945454a5494248046535c70460/src/mongo/db/repl/replication_recovery.cpp#L244-L251

I believe that this fassert used to be expected to be hit in MongoDB 4.0, but not in 4.2 and later. Let me know if the fassert isn't expected to be hit in 4.0 as well and I'll file another bug report

Here is the fassert in 4.0: https://github.com/mongodb/mongo/blob/6156b718630344212bea269a28ed5bacf96a9b86/src/mongo/db/repl/replication_coordinator_impl.cpp#L789