I tried with org-mode version 9.0.10 and 9.0.1.
There is still no filtering when I do ^ in org agenda mode when the point
is anywhere on a line where there is a task.

I observe that:
- The version 9.0.1 dates to November 2016.
- org-agenda-filter-by-top-headline is a very useful function.

It probably means that if it was a bug it would probably have been fix by
then. But it is not. Which makes me think it is probably not a bug and that
I am probably not using this function the right way.

How do I filter the tasks listed in the agenda list by top headline?

Regards.