I'm seeing the same issue.

The documentation describes the process clearly: https://developers.facebook.com/docs/facebook-login/guides/%20access-tokens/debugging

But trying to hit that endpoint with the input_token from a user who has authorized my app and the access_token from my Threads app like so:

https://graph.facebook.com/debug_token?input_token=THQxxxnU0&access_token=1929xxxbf3

I always get an error like this:

Response: {"error":{"message":"(#100) The App_id in the input_token did not match the Viewing App","type":"OAuthException","code":100,"fbtrace_id":"ASxxx2s"}}

Is there a different endpoint to use for debugging Threads app tokens, or additional credentials required not shown in the documentation?