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 
if (!spk_man) {
        return false;
    }
    LOCK(spk_man->cs_KeyStore);
    return spk_man->ImportPrivKeys(privkey_map, timestamp);
}
bool CWalletImportPubKeys(const stdvector<CKeyID>& ordered_pubkeys, const stdmap<CKeyID, CPubKey>& pubkey_map, const stdmap<CKeyID, stdpair<CPubKey, KeyOriginInfo>>& key_origins, const bool add_keypool, const bool internal, const int64_t timestamp)
{
    auto spk_man = GetLegacyScriptPubKeyMan();
    if (!spk_man) {
        return false;
    }
    LOCK(spk_man->cs_KeyStore);
    return spk_man->ImportPubKeys(ordered_pubkeys, pubkey_map, key_origins, add_keypool, internal, timestamp);
}
bool CWalletImportScriptPubKeys(const stdstring& label, const stdset<CScript>& script_pub_keys, const bool have_solving_data, const bool apply_label, const int64_t timestamp)
{
    auto spk_man = GetLegacyScriptPubKeyMan();
    if (!spk_man) {
        return false;
    }
    LOCK(spk_man->cs_KeyStore);
    if (!spk_man->ImportScriptPubKeys(script_pub_keys, have_solving_data, timestamp)) {
        return false;
    }
    if (apply_label) {
        WalletBatch batch(GetDatabase());
        for (const CScript& script  script_pub_keys) {
            CTxDestination dest;
            ExtractDestination(script, dest);