thread::join failed: Invalid argument for c++20 std::jthread

I'm using c++20's std::jthread (I'm actually using apple-clang 14.0 so I'm relying on Nicolai Josuttis' implementation).

In the program below I'm unable to join the default-constructed std::jthread thread with a thread.join() at the end of do_stuff() and the program fails with the following output:

Starting thread...
Max attempts exceeded.
libc++abi: terminating with uncaught exception of type std::__1::system_error: thread::join failed: Invalid argument

#include <fmt/core.h>
#include <thread>

class ThreadedProcessor {

private:
    int attempt{0};
    const int max_attempts = 1;
    std::stop_source ssource;
    std::jthread thread;

    void do_stuff() {
        while (attempt < max_attempts) {
            ++attempt;
            std::this_thread::sleep_for(std::chrono::milliseconds(100));
        }
        fmt::print("Max attempts exceeded.\n");
        ssource.request_stop();
        thread.join();
    }

    void start() { // was previously called process()
        thread = std::jthread([&, stoken = ssource.get_token()]() {
            fmt::print("Starting thread...\n");
            while (!stoken.stop_requested()) {
                do_stuff();
            }
            if (stoken.stop_requested()) {
                fmt::print("Stop requested. Stopping thread.\n");
            }
        });
    }

public:

    ThreadedProcessor() {
        process();
    }
};

int main() {
    auto tp = ThreadedProcessor();
}

How does one stop the thread properly in this case?