As the issue is gone with the 3.10 universal2 installer it should also be gone with the 3.9.1 universal installer. I'd expect that the difference is in the use of a newer version of Tcl/Tk. 

We haven't used the newer versions of Tcl/Tk with the regular 3.9 installer because we (or rather Ned) have run into various issues when building recent versions of Tcl/Tk on ancient versions of macOS. The universal2 installer is build on macOS 11 and hence avoids those build issues.  

Testing if Tcl/Tk 8.6.11 works correctly when build on macOS 10.9 could be an interesting experiment if someone is up to this, but is something I don't intend to do.  I rarely use Tkinter myself and this requires manual testing to verify behaviour.