I checked with one of our developers, and he says that as far as he's aware, what you've described should work in stable/2.6.x. I see the GitHub link you shared is for 2.6.4, but is there any chance that you're using the qa/2.x branch locally? There are some changes there that might affect your implementation.
Otherwise, a couple other things to try:
Rebuild the theme CSS?
- make -C plugins/[your plugin name here]
Note that if you installed locally using the tarball, you may need to first install the dependencies required to run this command - we precompile the CSS in the tarball releases, so the dependencies aren't included. Instructions can be found in the last part of the Option 2 installation instructions, here:
Try re-running the cache:clear and PHP-FPM restarts - and also ensure that you clear your browser cache as well, or else test in a private or incognito browser window, where the browser cache is typically disabled by default?
Hopefully one of these things will help it work as expected.