I work in HR and do the benefits for ~700 people, and unfortunately this is a common issue. As fate would have it, we have Anthem BC/BS.
Others are correct, somewhere along the line, there was a clerical issue.
If that issue wasn't corrected (sometimes they send it to HR, sometimes they send it to the user), it is terminated within 30 days. So, it sounds like there was an enrollment issue, and somewhere someone didn't follow procedure, and you're the recipient of the fall out.
Rest assured- it can be back dated to cover whatever issues occurred during this, up to 30 days. Don't let them tell you it can't be fixed.