I been having the same problem for maybe the past 2-3 weeks.
The fast forward preview pane is not working when using the Netflix app, but appears to be OK with most, not all, other apps.
I called Netflix, and after 40 minutes was told that it was an Apple problem and that their engineering staff have submitted a "ticket" to Apple to get a fix.
I tend to agree with what others have said about this being an App issue that falls in the Netflix court, but wasn't going to spend time arguing with them.
Maybe the best thing users can do is continue to call Netflix support, (866) 579-7172 to escalate the urgency to get it fixed.
Apple TV4
tvOS 11.0