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 
UniValue warnings(UniValue::VARR);
    UniValue result(UniValue::VOBJ);
    try {
        if (!data.exists("desc")) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Descriptor not found.");
        }
        const std::string& descriptor = data["desc"].get_str();
        const bool active = data.exists("active") ? data["active"].get_bool() : false;
        const bool internal = data.exists("internal") ? data["internal"].get_bool() : false;
        const std::string label{LabelFromValue(data["label"])};
        // 
        FlatSigningProvider keys;
        std::string error;
        auto parsed_desc = Parse(descriptor, keys, error, /* rue);
        if (!parsed_desc) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, error);
        }
        // 
        int64_t range_start = 0, range_end = 1, next_index = 0;
        if (!parsed_desc->IsRange() && data.exists("range")) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Range should not be specified for an un-ranged descriptor");
        } else if (parsed_desc->IsRange()) {
            if (data.exists("range")) {
                auto range = ParseDescriptorRange(data["range"]);
                range_start = range.first;
                range_end = range.second + 1; // 
            } else {
                warnings.push_back("Range not given, using default keypool range");
                range_start = 0;
                range_end = wallet.m_keypool_size;