Truncating String.IO() won't take affect

Question

So I am making a flask app to build team configurations using a backtracker. Everything on the backend is fine but I am having trouble serving the content. This is not a production thing, just a script for me and my friends. I capture stdOut into a StringIO() object. I then write that to a file and serve the file to the browser. The issue is that somewhere, the buffer is not being cleared and I get repeat data on each request.

EDIT: I capture STDOUT so I can keep the original implementation of the backtracker. IE: Running the backtracker outputs to the terminal, running the app will run the behavior below. Is this good design principles?

@app.route('/<solutions>', methods=['GET'])
def getTeams(solutions=1):
    f = open("tmp.txt", "w")


    solutions = int(solutions)

    old_stdout = sys.stdout
    old_stdout.flush()
    sys.stdout = mystdout = StringIO()

    teams.main(solutions)

    sys.stdout = old_stdout
    f.write(mystdout.getvalue())
    f.close()

    mystdout.seek(0)
    mystdout.truncate(0)


    return send_file("tmp.txt")

It works fine, the file clears every run, but the outputs to the webpage are as follows: First Response

Starting backtracker...
Printing 1 solutions...

          TEAM 1                  TEAM 2
 -------  ----------------------  -------------------
 TOP      Name: BilyBobLongSlong  Name: auzzi29
 MID      Name: balberry13        Name: ZeSwagerWagon
 JUNLGE   Name: Lemme Dunk First  Name: MackAttack599
 ADC      Name: RINK              Name: sptfire128
 SUPPORT  Name: JCKing            Name: rohan72699

Second Response:

Starting backtracker...
Printing 1 solutions...

          TEAM 1                  TEAM 2
 -------  ----------------------  -------------------
 TOP      Name: BilyBobLongSlong  Name: auzzi29
 MID      Name: balberry13        Name: ZeSwagerWagon
 JUNLGE   Name: Lemme Dunk First  Name: MackAttack599
 ADC      Name: RINK              Name: sptfire128
 SUPPORT  Name: JCKing            Name: rohan72699

          TEAM 1                  TEAM 2
 -------  ----------------------  -------------------
 TOP      Name: BilyBobLongSlong  Name: ZeSwagerWagon
 MID      Name: auzzi29           Name: MackAttack599
 JUNLGE   Name: balberry13        Name: JCKing
 ADC      Name: sptfire128        Name: RINK
 SUPPORT  Name: Lemme Dunk First  Name: rohan72699

and every response it just stacks more on. I don't quite know why the StringIO() will not clear as I seek it to 0 and truncate it every run of the code... Am very confused. Thanks!

Update: I have checked the docs on StringIO and it is supposed to behave as a file. So I'm lost. I also print() what is in StringIO to the command line to outrule the possibility of some weird interaction with the temporary file and as suspected, printing out StringIO is the same un-truncated output.

Could this be some kind of bug? I don't understand where the output could be persisting.... hmmm....