Keyboard and mouse inop at filevault screen

The machine is an early 2015 13" MBP, 16GB RAM, i7, 512GB SDD, running OS X 10.10.5.

This machine has run flawlessly where this problem wasn't present until two days ago. When I reboot the machine and it prompts for the FileVault password, the keyboard and mouse are inoperable, where both are locked except for random 250-500 millisecond intervals where both work briefly, then go back to being unusable. Booting to the reset password screen works.

Even more odd is that clearing the NVRAM via option-command-P-R also alleviates the problem until the next startup.

This doesn't seem like a hardware issue, and it might be a recent OS X update, but it is a pretty odd bug. I initially reinstalled OS X (as a file related to Core Storage was corrupted as per the Fix Permissions dialog in the Disk Utility), but this continues to persist. Without FileVault on, everything is as normal.

Are there any known bugs or workarounds for this?