Closed captioning issues on Amazon Fire TV can be caused by several factors, such as incorrect settings, outdated software, and compatibility issues with the video or streaming service being used. To resolve the issue, try the following steps:
# Check the closed captioning settings on your Amazon Fire TV device to ensure they are turned on and set to the correct language.
# Make sure your Amazon Fire TV software is up to date by going to "Settings" and then "Device" and checking for software updates.
# Try disabling and re-enabling closed captioning on your Amazon Fire TV device to see if this resolves the issue.
you can also get solutions for this on :
https://developer.amazon.com/docs/fire-tv/faq-general.html?tag=ifixitam-20
https://www.firesticksolutions.com/firestick-closed-caption-wont-turn-off/