Did you try calling them? I also have student loans that were transferred to Cornerstone. I fell behind back in 2016. Unfortunately it's split as 3 loans, so each reports as having late payments. They have since been paid off, but are still reporting late.
Trying to rebuild myself, so I sent my first GW letter to them last month with no success. I planned on sending another one and trying to call them and speak to someone next week. Just wondering if you tried and/or had any success.
I believe that Cornerstone is a servicer of federal student loans.
Provisions of the Higher Education Act mandate the reporting of delinquencies on certain types of federal loans, removing the discretion of good-will deletions.
You should first determine if your loan is a type that is covered by mandatory reporting under the Higher Education Act.