Why does English subtitle show up when English speaking dialogue is spoken on Apple TV streaming?

If your settings for subtitles are are set to English, then that will show whenever that is available in the stream.

If your settings for closed captions are on, then that will show whenever that is available in the stream.

What is your model Apple TV? Identify your Apple TV model - Apple Support

This is a user forum, you are not addressing apple here  just  fellow users

 maybe by rules of the content provider or maybe by choice of the content creators

you can try contacting apple Support ->  Get Support (apple.com)

and or give them feedback -> Product Feedback - Apple