qtbot causes crash in xfwm4

I am using qtbot for a while now with some (<10) testcases. I noticed that I have to restart my window manager 2 out of 5 test runs. That is why I have a console open where I can restart the window manager.
I just got the following error in the console while running the tests:

(xfwm4:8437): Gdk-ERROR **: 18:46:15.395: The program 'xfwm4' received an X Window System error.
This probably reflects a bug in the program.
The error was 'XSyncBadCounter'.
  (Details: serial 5287398 error_code 134 request_code 134 (SYNC) minor_code 3)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the GDK_SYNCHRONIZE environment
   variable to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)

Probably you can not do anything since you are using a framework or something. Please just let me know if this is a known issue or if I should report this upstream and where.
Thank you!

Start by reproducing the reported qtbot test runs under xfwm4 and review the XSyncBadCounter error from the issue. Determine whether the crash is caused by pytest-qt or an upstream window-manager/framework component, then document the responsible project and reporting path.