No idea. I would have suggested re-updating it on the PC side. I had to do that with my Logitech G600 a few weeks ago. Not 100% sure why. Same behavior as yours. Plugged it back into the PC and made sure the programming was right. Plugged back into PS4 and the side buttons were back in action.

I know PS4 did a 2.0 update a few weeks back. Not sure if that has anything to do with it, but the update does coincide with when I had to do my G600 update.