I have had several complaints about this. Its not just C4, its URC too. I also notice it on the new ATV and the new 4k unit too. When you are streaming, it wont FF and REW good.

I have found if you set iTunes to download the movie to your computer, then stream from the computer to the AppleTV the commands work fine. But you need to make sure you have your computer not set to sleep, and it has to be stored on the main HDD.

If you put it on a NAS or external, Sierra and High Sierra wont automount external drives. I had to build a script to automount some NAS drives because of this. If the drive doesn't automount, then iTunes will use the main HDD/iTunes as the default folder, then you wont have access to your movies. Then it streams from the web again and you can't FF and Rew.

This has become a royal pain, and I am telling people to start using Amazon because iTunes and AppleTV just isn't a reliable source for media anymore. These glitches were supposed to be fixed with High Sierra, and they were not. Instead of fixing glitches they just keep rolling out new product.