As far as I recall, we have only ever shipped 8.6.8 in final releases since we started providing a built-in Tcl/Tk in the python.org macOS installers as of 3.7.0, 3.6.8, and 2.7.16 (https://www.python.org/download/mac/tcltk/).  There was the disastrous attempt to update to 8.6.9+ during the 3.7.2rc phase which we ultimately reverted prior to 3.7.2 final, as documented in Issue35485.  Since then, there do not seem to have been any official releases of 8.6.9 so we remain with 8.6.8.  It would probably be a good thing for someone to work with the Tk folks on seeing whether the problems seen are being resolved but it's not something I expect to have time for in the foreseeable future.