Why is there set-cookie in every response?

Even though there is a cookie in the request, there is a set-cookie in each response.
Why not only set-cookie in the first response?
Thank you；
this my koa-session config:

The report names only a koa-session configuration shown in an attached screenshot; it names no source file, test, or entry point. Reproduce the repeated Set-Cookie behavior from that configuration, then determine whether the intended outcome is an explanation, documentation, or a code and test change.

Written by the indexing model from the issue text.