It looks like there are two things happening: First, your FireTV stick may not support the video format; Second, there is a problem with an Intel driver affecting hardware transcoding.
First, the transcoding:
Your video is HEVC Main 10. Is it 10-bit? Check Get Info if you are not sure.
You have the Fire TV Stick Basic Edition. It does not support 10-bit HEVC video, so Plex is transcoding it to H.264, 8-bit video.
Fire TV Stick Basic Edition Tech Specs
Second, the distorted picture.
On which OS and CPU are you running PMS?
As mentioned, there is a problem with an Intel supplied driver and hardware transcoding on systems with Apollo Lake CPUs. Plex is working with Intel to resolve the problem.
If this is the same problem you are experiencing, the easiest workaround is to disable hardware accelerated streaming.
You can also delete the offending driver. Plex will then fall back to an older driver.
This post has info on how to delete the driver.
