Thank you Larry.  It can never be too careful to deal with language issues!

> why the code behaves like this--is this behavior a genuine bug, or is it actually a bugfix for some worse behavior?

In my view, there's rather an concept needing clarify, than a genuine bug.  I look into the file blame, and it shows the processing order around `follow_wrapper_chains` might be carefully arranged...

- The `follow_wrapper_chains` functionality is added in issue 13266: https://bugs.python.org/issue13266
- When `signature()` first implemented, `__wrapped__` is handled then `partial`, this order never changed.
   - Issue #15008: Implement PEP 362 "Signature Objects": https://github.com/python/cpython/commit/7c7cbfc00fc8d655fc267ff57f8084357858b1db

> will fixing the bug cause problems for Python users? and if so, can we still fix the bug while mitigating the damage to people who are unfortunately depending on the bug?

It's really a heavy responsibility!  Slow down is right... A clear explaination and plan should be constructed before any action is taken.

So I may study PEP 362 to get some context, about why the code is this way.  And I'm wondering what else can I do?