OnePlus co-founder Carl Pei stated that the update is rolled back because they needed to include the “Ok OnePlus” feature. Anyhow, today it would appear that the Cyanogen OS 12 update rollout was ceased again to fix some “critical” bugs. As per a post on the company’s discussions:
Just FYI, the reasons why we stopped the initial rollout are because of a problem updating encrypted devices from 11 to 12, and a couple of bugs that we felt were critical enough to fix before continuing (mms/telephony/themes). We also had an error in our production build environment that broke voice wakeup.
OnePlus One users who have already installed the update seasoned Cyanogen OS 12 will get a small update that will fix the existing bugs, also includes the new “Ok OnePlus” feature.