I have two AppleTV devices, and I can easily preview through FF and RW when playing the same programs.
That's interesting. On a Roku, it's up to the provider to supply those images. I don't know how the other devices work. I'm pretty sure I've read in the Roku certification requirements that the trick play scenes (or BIF files) are required, but Roku has a way of letting some channels slide. There's no setting or anything like that affects them, but you might want to reach out to Spectrum and ask them why they don't provide them.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.