Why can't I send arguments as reference to thread function?

Question

I have the next code:

#include <iostream>
#include <Windows.h>
#include <thread>
#include <string>

void func(std::string& str)
{
    std::string to_change("Bye...");
    std::cout << "From func: " << str << std::endl;
    str.swap(to_change);
}

int main()
{
    std::string Meeting = "Hi!";
    std::thread t1(func, Meeting);
    t1.join();

    std::cout << "From main: " << Meeting << std::endl;


    Sleep(20000);
    return 1;
}

The output is:

From func: HI

From main: HI

But if I do this type of moving by reference:

std::ref(Meeting)

Then the output goes:

From func: HI

From main: Bye...

What are the difference between sending the both reference types?