I experience the same issue on both my Windows and Linux systems. It seems the feature hasn't been fully implemented yet.
Remember, the disclaimer at the top of the
about:flags page reads:
Careful, these experiments may bite
WARNING These experimental features may change, break, or disappear at any time. We make absolutely no guarantees about what may happen if you turn one of these experiments on, and your browser may even spontaneously combust. Jokes aside, your browser may delete all your data, or your security and privacy could be compromised in unexpected ways. Please proceed with caution.
Just feel lucky your browser didn't spontaneously combust! :-)