Logging in Every Minute - Craft Admin Sessions (on Heroku)
My own answer... for anyone interested, the Memcached extension in my composer.json file wasn't being picked up by Heroku on deploy. So I updated my composer.json file like this:

{
  "require": {
    "ext-mbstring": "*",
    "ext-imagick": "*",
    "ext-memcached": "*"
  }
}


Then ran composer update to update the composer.lock file.

When deploying to Heroku, the composer.lock file is very similar to the Gemfile.lock in Rails - telling Heroku exactly what to install with the deployment. Now the Memcached extension is being installed by Heroku and my session problem has been solved.