Hello @Aditya Aranya ,
Thanks for the question and using MS Q&A platform.
If I understand correctly. the copy activity fails - when the pagination happens.
If there is a long duration for copy activity - for first iteration to complete. There is a possibility the token getting expired before the second request.
This is causing a 401 - authentication issue.
Note :
You could quickly check your token here - https://jwt.ms - decoded.
The expiry is in epoch, and you could convert the epoch time here. See whether the error is happening after this time.