Run locally with `g++ bug.c && ./a.out` or run online at https://coliru.stacked-crooked.com/a/785c49eb44c427fc

bug.c

#include <iostream>
#include <string>
 
int main()
{
    std::string data = "test";
    std::string search = "";
    std::string replace = "";
    
    std::cout << "data = «" << data << "» (size = " << data.size() << ")\n";
    std::cout << "search = «" << search << "» (size = " << search.size() << ")\n";
    std::cout << "replace = «" << replace << "» (size = " << replace.size() << ")\n";

    std::cout << "----\n";

    std::size_t pos = data.find(search);

    std::cout << "pos = " << pos << "\n";
    
    std::cout << "pos == npos? ";
    if (pos == std::string::npos)
        std::cout << "yep\n";
    else
        std::cout << "nope\n";

    std::cout << "----\n";

    std::cout << "replace(" << pos << ", " << search.size() << ", \"" << replace << "\")\n";
    data.replace(pos, search.size(), replace);
    std::cout << "data = «" << data << "» (size = " << data.size() << ")\n";
    
    std::cout << "----\n";
    
    pos = data.find(search, pos + replace.size());

    std::cout << "pos = " << pos << "\n";
    
    std::cout << "pos == npos? ";
    if (pos == std::string::npos)
        std::cout << "yep\n";
    else
        std::cout << "nope\n";
}