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 (involvesWatchonly || (wallet.IsMine(r.destination) & ISMINE_WATCH_ONLY)) {
                entry.pushKV("involvesWatchonly", true);
            }
            MaybePushAddress(entry, r.destination);
            PushParentDescriptors(wallet, wtx.tx->vout.at(r.vout).scriptPubKey, entry);
            if (wtx.IsCoinBase())
            {
                if (wallet.GetTxDepthInMainChain(wtx) < 1)
                    entry.pushKV("category", "orphan");
                else if (wallet.IsTxImmatureCoinBase(wtx))
                    entry.pushKV("category", "immature");
                else
                    entry.pushKV("category", "generate");
            }
            else
            {
                entry.pushKV("category", "receive");
            }
            entry.pushKV("amount", ValueFromAmount(r.amount));
            if (address_book_entry) {
                entry.pushKV("label", label);
            }
            entry.pushKV("vout", r.vout);
            if (fLong)
                WalletTxToJSON(wallet, wtx, entry);
            ret.push_back(entry);
        }
    }
}
static std::vector<RPCResult> TransactionDescriptionString()