cache busting is lacking since Plone 5.1
get it from an older setup; you can go to your buildout-cache directory under the Products/CMFPlone/static/components/less/dist directory
then grab the 5.0 version like this:
same thing for the less.min.js (not sure if it's used)
Things should work better with this old version (not perfect, there are still some annoying quirks such as a strange popup after compilation, and also don't forget to save modified less files before compiling) but at least it works, without it trying to use this function is complete madness like you have seen for yourself.