I just noticed this today, I'm assuming it's from a recent update.
The previous behavior was that when you clicked a modified file in the "N files changed" dropdown under the checked out repository in the Repository browser, it opened a diff view of those unstaged changes ( which I believe is comparing the working tree to the index ). This allowed you to select ranges to stage or view what was being changed as you staged files for commit.
Now, it opens a diff view between the working tree and the working tree, which as you guessed, does not display any changes because it's comparing the same things.
Tried to find configuration settings that would modify the behavior but could not. I asssume it's the result of a recent change in the last week or so as there are similar bugs resolved that presumably hit the same area of code.
Thanks!
Steps to Reproduce:

I see the same issue after upgrading to Gitlens 9.9.2
I see the same issue today
when I click '3 files changed', I expect to see what I changed for every line.
but for now, the unstaged files just compare with the unstaged files itself. therefore it shows nothing different
I guess is because of the newest update
VSCode Version: 1.36.1
I see the same issue recently.
@dsmythe @ajaypanyala @Jadson666 @nztraveler Sorry for the inconvenience -- this should be resolved in v9.9.3. Thanks for the report!
Yep, can confirm 9.9.3 resolved the issue now, Thanks @eamodio!
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Most helpful comment
@dsmythe @ajaypanyala @Jadson666 @nztraveler Sorry for the inconvenience -- this should be resolved in v9.9.3. Thanks for the report!