Flask hangs when run as a subprocess alongside pytest-CodePudding

I've spent the last hour and a half trying and failing to debug this test and I am utterly stumped. To simplify the process of testing the Flask server I am building, I have made a relatively simple script which starts the server, then runs pytest, kills the server, writes the outputs to files, and exits with Pytest's exit code. This code was working perfectly until today, and I haven't modified it since (aside from debugging this issue).

Here's the problem: when it gets to a certain point in the tests, it hangs. The weird thing is that this does not happen if I run my tests in any other way.

Debugging my server in VS Code, and running tests in the terminal: works
Running my server using the same code used in the test script and running pytest manually: works
Running pytest using the test script and running the server through the start server script (which uses the same code for running the server as the test script does) in a second terminal: works

Here's the other interesting thing: the tests always hang in the same place, part way through the setup fixture. It sends the clear command, and an echo request to the server (which prints the name of the current test). The database clears successfully, and the server echoes the correct information, but the echo route never exits - my tests never get a response. This echo route behaves perfectly for the 50 or so tests that happen before this point. If I comment out the test that is causing it to fail, it fails on the next test. If I comment out the call to the echo then it hangs on a later test on a completely different request to a different route. When it hangs, the server cannot be killed using a SIGTERM, but instead requires a SIGKILL.

Here is my echo route:

@debug.get('/echo')
def echo() -> IEcho:
    """
    Echo an input. This returns the given value, but also prints it to stdout
    on the server. Useful for debugging tests.

    ## Params:
    * `value` (`str`): value to echo
    """
    try:
        value = request.args['value']
    except KeyError:
        raise http_errors.BadRequest('echo route requires a `value` argument')

    to_print = f'{Fore.MAGENTA}[ECHO]\t\t{value}{Fore.RESET}'
    # Print it to both stdout and stderr to ensure it is seen across all logs
    # Otherwise it could be more difficult to figure out what's up with server
    # output
    print(to_print)
    print(to_print, file=sys.stderr)
    return {'value': value}

And here is my code that sends the requests:

def get(token: JWT | None, url: str, params: dict) -> dict:
    """
    Returns the response to a GET web request

    This also parses the response to help with error checking

    ### Args:
    * `url` (`str`): URL to request to

    * `params` (`dict`): parameters to send

    ### Returns:
    * `dict`: response data
    """
    return handle_response(requests.get(
        url,
        params=params,
        headers=encode_headers(token),
        timeout=3
    ))

def echo(value: str) -> IEcho:
    """
    Echo an input. This returns the given value, but also prints it to stdout
    on the server. Useful for debugging tests.

    ## Params:
    * `value` (`str`): value to echo
    """
    return cast(IEcho, get(None, f"{URL}/echo", {"value": value}))

@pytest.fixture(autouse=True)
def before_each(request: pytest.FixtureRequest):
    """Clear the database between tests"""
    clear()
    echo(f"{request.module.__name__}.{request.function.__name__}")
    print("After echo")  # This never prints

Here is my code for running Pytest in my test script

def pytest():
    pytest = subprocess.Popen(
        [sys.executable, '-u', '-m', 'pytest', '-v', '-s'],
    )

    # Wait for tests to finish
    print("