Just want to add that I've gone through this issue with my mother for the Prime app on her xfinity box. It is VERY finicky. Sometimes the subtitles appear, sometimes they don't, regardless of whether they are selected at the show/movie level (from the Subtitles menu item.) In one case, we had subtitles turned on, but they would not appear. Clicked the select button (Ok on the xfinity remote,) scrolled right to CC and saw that the language was default. Used that menu to turn subtitles off and then on again. Now it said English and lo and behold, they appeared.
So keep working with the various controls for CC/subtitles on the Comcast xfinity app and you will be able to make them appear. Just wish they had a global setting for this that would work without fail.
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