New Apple TV remote doesn’t fast forward on YouTube app. I don’t have that issue with my older Apple TVs remotes.

Control what’s playing on Apple TV - Apple Support

You have three methods available, and two out of three are supported in the YouTube app.

• Skip backwards or forwards 10 seconds, by pressing left or right.
• Continuously rewind or fast-forward, by using press-and-hold left or right (not supported in the YouTube app).
• Scrub the video timeline: first pause, move the playhead to a different position in the video, then continue.

Apps that use their own video player instead of the standard tvOS video player, will have to program how to react to remote input. That means that only the YouTube programmers can add functionality to match what you are used to. How to contact an app developer - Apple Support