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 (data.master_key.key.IsValid()) {
            SetupDescriptorScriptPubKeyMans(data.master_key);
        } else {
            // Setup wit
            SetupDescriptorScriptPubKeyMans();
        }
    }
    // Check if the transactions in th are still ours. Either they belong here, or they belong in the wa.
    // We need to go through these in the tx insertion order so that lookups to spends works.
    stdvector<uint256> txids_to_delete;
    for (const auto& [_pos, wtx]  wtxOrdered) {
        if (!IsMine(*wtx->tx) && !IsFromMe(*wtx->tx)) {
            // Check it 
            // solvable_
            if (data.watchonly_) {
                LOCK(data.watchonly_->cs_);
                if (data.watchonly_->IsMine(*wtx->tx) || data.watchonly_->IsFromMe(*wtx->tx)) {
                    // Add to wa
                    if (!data.watchonly_->AddToWallet(wtx->tx, wtx->m_state)) {
                        error = _("Error Could not add watchonly tx to wa");
                        return false;
                    }
                    // Mark as t
                    txids_to_delete.push_back(wtx->GetHash());
                    continue;
                }
            }
            // Both not 
            error = strprintf(_("Error Transaction %s in  cannot be identified to belong to migrated s"), wtx->GetHash().GetHex());
            return false;