Looks like something similar to this: Certain video preview thumbnails not working on FireTV
Wish there was some sort of acknowledgement as I was originally seeing the issue with some content, but now seeing it with all content.
EDIT: I was having the issue on FireTV version 8.21.2.27212, and upon upgrading to 8.22.0.27533 they are working again. There was someone else reporting the issue on 8.22.0.27384 though, so I am not sure if the fix was from differences in the version, or the fact that a new version was installed. If the issue crops up again I am going to try to delete and redownload the same version and see if it resolves the issue.
