So I bought a brand new Retina MacBook Pro 15" Mid-2014 model with all the fixins, enabled FileVault during system setup, and got this same problem.
I didn't do the NVRAM and PRAM reset. Guided by 10+ years of IT experience working on both PCs and Apple products, I decided it was better to just start over before I got too invested in my install because there's a good chance the half-enabled FileVault encryption would have nerfed the system before too long.
This is how I fixed the problem. FileVault is now working perfectly for me. Don't do this before backing your files up if you care about losing them.
Step 1: Power off the system.
Step 2: Hit the power button to turn the system on.
Step 3: As the system is just powering on (like... right after you hear the Apple gong), hold Command + Control + R to enter the Internet-based recovery. This is NOT THE SAME as the Command + R recovery mode. It loads the recovery software from the internet and gives you more freedom in Disk Utility.
Step 4: Open Disk Utility.
Step 5: Click on the system partition, click on File, and click Unlock the drive.
Step 6: Once the drive is unlocked, click on the partition, go to the Erase menu, and erase the partition with a Mac OS Extended (Journaled) partition. DO NOT SELECT THE Mac OS Extended (Encrypted, Journaled) option. You want to make a fresh, decrypted partition.
Step 7: Exit Disk Utility and install the operating system. It'll take about two-ish hours to download and install the new OS, so put on a pot of coffee and pick a show to marathon on Netflix while you wait.
Step 8: Do not activate FileVault encryption while you're setting up the computer for first-time use. Once that setup is complete and the computer is up and running, go into System Preferences, into Security & Privacy, and turn on FileVault. From that point, do absolutely nothing with the machine until it finishes encryption.
Good luck, folks.