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 
}
    return res;
}
bool CWalletApplyMigrationData(MigrationData& data, bilingual_str& error)
{
    AssertLockHeld(cs_);
    LegacyScriptPubKeyMan* legacy_spkm = GetLegacyScriptPubKeyMan();
    if (!legacy_spkm) {
        error = _("Error This  is already a  ");
        return false;
    }
    for (auto& desc_spkm  data.desc_spkms) {
        if (m_spk_managers.count(desc_spkm->GetID()) > 0) {
            error = _("Error Duplicate s created during migration. Your  may be corrupted.");
            return false;
        }
        m_spk_managers[desc_spkm->GetID()] = stdmove(desc_spkm);
    }
    // Remove the LegacyScriptPubKeyMan from disk
    if (!legacy_spkm->DeleteRecords()) {
        return false;
    }
    // Remove the LegacyScriptPubKeyMan from memory
    m_spk_managers.erase(legacy_spkm->GetID());
    m_external_spk_managers.clear();
    m_internal_spk_managers.clear();
    // Setup new s
    SetWalletFlag(WALLET_FLAG_DESCRIPTORS);
    if (!IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
        // Use the existing master key if we have it