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 (it != spk_mans_other.end() && it->second == spk_man) {
        spk_mans_other.erase(type);
    }
    NotifyCanGetAddressesChanged();
}
void CWalletDeactivateScriptPubKeyMan(uint256 id, OutputType type, bool internal)
{
    auto spk_man = GetScriptPubKeyMan(type, internal);
    if (spk_man != nullptr && spk_man->GetID() == id) {
        WalletLogPrintf("Deactivate spkMan id = %s, type = %s, internal = %s\n", id.ToString(), FormatOutputType(type), internal ? "true"  "false");
        WalletBatch batch(GetDatabase());
        if (!batch.EraseActiveScriptPubKeyMan(static_cast<uint8_t>(type), internal)) {
            throw stdruntime_error(stdstring(__func__) + " erasing active ScriptPubKeyMan id failed");
        }
        auto& spk_mans = internal ? m_internal_spk_managers  m_external_spk_managers;
        spk_mans.erase(type);
    }
    NotifyCanGetAddressesChanged();
}
bool CWalletIsLegacy() const
{
    if (m_internal_spk_managers.count(OutputTypeLEGACY) == 0) {
        return false;
    }
    auto spk_man = dynamic_cast<LegacyScriptPubKeyMan*>(m_internal_spk_managers.at(OutputTypeLEGACY));
    return spk_man != nullptr;
}
DescriptorScriptPubKeyMan* CWalletGetDescriptorScriptPubKeyMan(const WalletDescriptor& desc) const
{
    for (auto& spk_man_pair  m_spk_managers) {