In short, we could get in a state where `IsReplica` could be true, but we couldn't use it because it was unselectable via this path, especially on a reconnect race. This caused failures in other Sentinel testing, specifically the `NoConnectionAvailable` variety on `DemandMaster` sanity check gets, because we had a replica, it was connected, but the flags said we couldn't use it. That should _never_ be the case and this additional check at the flag set site ensures it.