Strange error when starting Matlab from Python via Native Messaging Extension (Firefox)

Question

I am trying to run some Matlab Code on an URL that I retrieve from a Firefox Native Messaging app. I use a python script that uses the Matlab engine to run my scripts. This works perfectly fine:

import matlab.engine
import sys
import struct
import time
import subprocess
import logging

# Function to receive Message
def getMessage():
    rawLength = sys.stdin.buffer.read(4)
    print(rawLength)
    if len(rawLength) == 0:
        sys.exit(0)
    messageLength = struct.unpack('@I', rawLength)[0]
    message = sys.stdin.buffer.read(messageLength).decode('utf-8')

    eng = matlab.engine.connect_matlab()
    eng.workspace['url'] = message
    eng.eval('myMatlabFunc(url)', nargout=0)


getMessage()

Now I want to check if Matlab is running and if not start it:

if not matlab.engine.find_matlab():
    subprocess.run('C:\\Program Files\\MatlabR2020b-64bit\\bin\\matlab.exe')

This works and I can run my code afterwards as well:

eng = matlab.engine.connect_matlab()
eng.workspace['var1'] = '1'

But then I encounter a strange behaviour, that I cant explain.

As soon as the script is finished, Matlab exits without a Warning/Crash/Exception... The weirdest part is, that this only happens if I run the skript via the native messaging connection.

If I run the script in Pycharm, everythin works as expected.

I hope someone here can help me figure this out!

Thanks, Ben