C++ std::endl vs "\n"

I have a thread safe print function that uses lock_guard and std::cout and is called from different threads. When I replaced std::endl expressions with "\n", my program stopped printing to console. The main difference is that std::endl does two things: it inserts a newline character AND flushes the output buffer, while "\n" only inserts a newline character. In a multithreaded environment, without flushing, the output might stay in the buffer and not be visible immediately.