Depending on which account is retrieved the v3 API to retrieve accounts will sometimes return nicely formatted JSON object and sometimes it will retrieve a giant string. It’s the same API call though, just a different offset in the URI. Also if any of the accounts that I’ve had trouble with are bundled in a large call with accounts that are fine on their own then it corrupts the entire response by returning one gigantic string.
You mention certain accounts. Is there anything about the data for these accounts that is different than other accounts like use of special characters in their data?