Outline tool: unable to delete objects ONLY outside artboard

Dear Community,

I have created the following example file:

And here is the layers panel:

I want to remove parts of the polar grid falling outside of the artboard.

So, I select all the objects, go to Pathfinder and select Outline:

Then, I enable the stroke again for all the objects, go into Isolation mode, BUT when I try to delete parts of the objects falling outside of the artboard, I end up deleting parts of the objects falling also inside the artboard.

What am I doing wrong?

Thanks,

Alberto