Steps to reproduce the issue

1. Please use fresh install of the J! 3.6.2 with the sample data - "Learn Joomla - Sample Data"
2. Please find and open an article "Upgraders"

Expected result

Nothing should happen.

Actual result

After opening the article notification is displayed:
Warning
JUser: :_load: Unable to load user with ID: 123
in sample data, in this article in field 'modifed by' there is an ID of user - 123

System information (as much as possible)

Additional comments

We can see the same notice for categories, for example category "modules" also has this user ID in sample data in "modified by" filed.

I'm not sure if this a bug or a feature :) ?

In my opinion this notice should be displayed only if there is missing user who created article/category.
Can someone confirm this, or anyone knows if there were changes in core that cause this behaviour?

Thanks!
Tomasz