Thanks for reaching out. I solved the issue following @tcarroll2's solution from: https://github.com/pyenv/pyenv/issues/1375
The issue doesn't occur when using pyenv. It seems that the issue may have been caused by python sticking to the mac provided tkinter (version 8.5) instead of using a compatible version. With pyenv, I get python 3.7.4 and tkinter version 8.6.10 and it works flawlessly :)

Steps to reproduce crash: 
- Downloading Python version 3.8+ from python.org
- Typing python3 in terminal, importing tkinter as tk and starting a window with tk.Tk()
I am immediately logged out of my user. TkVersion is 8.6

Steps to reproduce weird window in window (my guess due to python using mac provided tkinter version 8.5):
- Downloading Python version 3.7.7 or older from python.org or homebrew
- Typing python3 in terminal, importing tkinter as tk and starting a window with tk.Tk()
- Voila! window in window created.

No logging out here, but the window created is unusable and glitchy. I tried running a tkinter GUI that works fine on other setups and the output was missing some widgets etc.