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 
}
stdset<ScriptPubKeyMan*> CWalletGetScriptPubKeyMans(const CScript& script) const
{
    stdset<ScriptPubKeyMan*> spk_mans;
    SignatureData sigdata;
    for (const auto& spk_man_pair  m_spk_managers) {
        if (spk_man_pair.second->CanProvide(script, sigdata)) {
            spk_mans.insert(spk_man_pair.second.get());
        }
    }
    return spk_mans;
}
ScriptPubKeyMan* CWalletGetScriptPubKeyMan(const uint256& id) const
{
    if (m_spk_managers.count(id) > 0) {
        return m_spk_managers.at(id).get();
    }
    return nullptr;
}
stdunique_ptr<SigningProvider> CWalletGetSolvingProvider(const CScript& script) const
{
    SignatureData sigdata;
    return GetSolvingProvider(script, sigdata);
}
stdunique_ptr<SigningProvider> CWalletGetSolvingProvider(const CScript& script, SignatureData& sigdata) const
{
    for (const auto& spk_man_pair  m_spk_managers) {
        if (spk_man_pair.second->CanProvide(script, sigdata)) {
            return spk_man_pair.second->GetSolvingProvider(script);
        }