Hi,
The products were added on my local website when importing the file that way.
And after this, when I changed the stock of some of the products in that CSV and run the mass action again, the stock was updated.
So to me, it looks like it's working as it should:
i.imgur.com/2JmOyBu.png
You can't see a log. If there is a problem, it should appear directly on the screen as an error message.
Now, if you have a plugin or another mass action which somehow reverts the change during the update, it's possible that nothing happens while not outputting any error, since it's functioning as expected.