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 
assert(spk_man != nullptr);
        for (const auto& keyid  GetAffectedKeys(scriptPubKey, *spk_man)) {
            WitnessV0KeyHash wpkh_dest(keyid);
            if (IsAddressUsed(wpkh_dest)) {
                return true;
            }
            ScriptHash sh_wpkh_dest(GetScriptForDestination(wpkh_dest));
            if (IsAddressUsed(sh_wpkh_dest)) {
                return true;
            }
            PKHash pkh_dest(keyid);
            if (IsAddressUsed(pkh_dest)) {
                return true;
            }
        }
    }
    return false;
}
CWalletTx* CWalletAddToWallet(CTransactionRef tx, const TxState& state, const UpdateWalletTxFn& update_wtx, bool fFlushOnClose, bool rescanning_old_block)
{
    LOCK(cs_);
    WalletBatch batch(GetDatabase(), fFlushOnClose);
    uint256 hash = tx->GetHash();
    if (IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE)) {
        // M
        stdset<CTxDestination> tx_destinations;
        for (const CTxIn& txin  tx->vin) {
            const COutPoint& op = txin.prevout;
            SetSKeyState(batch, op.hash, op.n, true, tx_destinations);
        }