This week's Engine Yard Cloud stack upgrade does not permanently correct the "Fixes issue so utility servers behind single app instances now use the correct values in memcached.yml (no longer point to localhost)" issue when you click the Upgrade button.
The stack upgrade corrects the issue if you have already added a utility instance to a solo instance and have the problem now. Clicking Upgrade fixes the issue (no need to also click Apply).
However, if you add a new utility instance at a later date, then you will need to click Apply for that utility instance at that time.
Click Apply each time you first deploy an app, or add a new utility instance.