It seems that it would otherwise silently skip the rescan for pre-backup used keys that are still waiting to be generated into the keypool?

See also #11249 
}
utilResult<CTxDestination> CWalletGetNewDestination(const OutputType type, const stdstring label)
{
    LOCK(cs_);
    auto spk_man = GetScriptPubKeyMan(type, /*internal
    if (!spk_man) {
        return utilError{strprintf(_("Error No %s addresses available."), FormatOutputType(type))};
    }
    auto op_dest = spk_man->GetNewDestination(type);
    if (op_dest) {
        SetAddressBook(*op_dest, label, "receive");
    }
    return op_dest;
}
utilResult<CTxDestination> CWalletGetNewChangeDestination(const OutputType type)
{
    LOCK(cs_);
    ReserveDestination reservedest(this, type);
    auto op_dest = reservedest.GetReservedDestination(tr
    if (op_dest) reservedest.KeepDestination();
    return op_dest;
}
stdoptional<int64_t> CWalletGetOldestKeyPoolTime() const
{
    LOCK(cs_);
    if (m_spk_managers.empty()) {
        return stdnullopt;
    }
    stdoptional<int64_t> oldest_key{stdnumeric_limits<int64_t>max()};
    for (const auto& spk_man_pair  m_spk_managers) {