** UPDATE Jan 27, 2021 **
I have been working with Microsoft on this issue and we did a request trace and reviewed the FREB log and confirmed that the code is outputting the correct headers but in the END_REQUEST handler they are replaced with cache-control private. After building several virtual machines from scratch we learned that out of the box, this problem doesn’t happen. However when we install the latest version of STACKIFY AGENT (4.29.29) once we put the agent on this behavior happens. Older versions of Stackify agent (RETRACE profiler and APM) don’t do this, but so far, this is irreversible: once we install 4.29.29 of Stackify, uninstalling or installing older versions doesn’t undo this issue, the environment is somehow permanently modified.