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 
{
    return RPCHelpMan{"walletpassphrasechange",
                "\nChanges the wallet passphrase from 'oldpassphrase' to 'newpassphrase'.\n",
                {
                    {"oldpassphrase", RPCArg::Type::STR, RPCArg::Optional::NO, "The current passphrase"},
                    {"newpassphrase", RPCArg::Type::STR, RPCArg::Optional::NO, "The new passphrase"},
                },
                RPCResult{RPCResult::Type::NONE, "", ""},
                RPCExamples{