I believe I have found bug in Magento 2.1.3
I have found the same issue at github and add a comment to it.
What is the best place to report issue?
Github or some other location?
Well for Magento Core Bugs - Yes you need to report issue on Github and you can share over here as well ! But from Github you will get exact solution for that issue !
If you found any new issue then you can report it over here - https://github.com/magento/magento2/issues
Click on the new issue button and generate the new issue on that repository !
Also as you mention that - the issue you found is already there on Github then yes you can just comment out over there only , so Members can get to know that Yes issue is still Exist !
Hope it helps !