Decryption of XOR-encrypted file aborts prematurely

Using a simple functor called Encryptor

struct Encryptor {
    char m_bKey;
    Encryptor(char bKey) : m_bKey(bKey) {}
    char operator()(char bInput) {
        return bInput ^ m_bKey++;
    }
};

I can easily encrypt this file using

std::ifstream input("in.plain.txt", std::ios::binary);
std::ofstream output("out.encrypted.txt", std::ios::binary);
std::transform(
    std::istreambuf_iterator<char>(input),
    std::istreambuf_iterator<char>(),
    std::ostreambuf_iterator<char>(output),
    Encryptor(0x2a));

however attempting to return this by calling

std::ifstream input2("out.encrypted.txt", std::ios::binary);
std::ofstream output2("out.decrypted.txt", std::ios::binary);
std::transform(
    std::istreambuf_iterator<char>(input2),
    std::istreambuf_iterator<char>(),
    std::ostreambuf_iterator<char>(output2),
    Encryptor(0x2a));

only works partially. Here are the file sizes:

in.plain.txt:      7,700 bytes
out.encrypted.txt: 7,700 bytes
out.decrypted.txt: 4,096 bytes

In this case, it seems that this method works only for the first 2**12bytes and probably only for its multiple (can it be my file system block size?). Why do I have this behavior and what is the workaround for this?