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 
#ifndef BITCOIN_WALLET_WALLET_H
#define BITCOIN_WALLET_WALLET_H
#include <consensus/amount.h>
#include <fs.h>
#include <interfaces/chain.h>
#include <interfaces/handler.h>
#include <logging.h>
#include <outputtype.h>
#include <policy/feerate.h>
#include <psbt.h>
#include <tinyformat.h>
#include <util/hasher.h>
#include <util/message.h>
#include <util/result.h>
#include <util/strencodings.h>
#include <util/string.h>
#include <util/time.h>
#include <util/ui_change_type.h>
#include <validationinterface.h>
#include <wallet/crypter.h>
#include <wallet/scriptpubkeyman.h>
#include <wallet/transaction.h>
#include <wallet/walletdb.h>