I use a Roku Ultra, and Amazon Prime subtitles are not displaying correctly on my TV. The default for Amazon Prime subtitles is white text with a black background. My Roku Ultra subtitles are also set for white text with a black background. When I watch Hulu or Netflix, the subtitles display correctly. When I watch Amazon Prime, they do not have the black background box. That makes it very difficult to read the white text when the show's background is white or a light color.
I have read that Roku cannot use Amazon Prime PRESET subtitles; that it can only use the DEFAULT subtitles. But it is not displaying the default setting. Also, the Amazon Prime subtitles DO display correctly when I'm watching on an iPad. So it seems to be a problem with the Roku/TV combination.