Steps to reproduce the issue

1. Create a category blog menu item which has articles assigned to the selected category.
2. Verify the category blog shows a list of articles.
3. Create a new tag (Components->Tags), but don't reference it on any article.
4. Edit the category blog menu item and assign the new tag, then save the menu item.
5. Verify that the category blog shows no articles (this is correct because the new tag is not assigned to the articles)
6. Edit the category blog menu item and remove the tag, then save the menu item.
7. Notice the category blog continues to show no articles (incorrect/unexpected).

Expected result

At step 7, because the tag was removed from the category blog menu item, it should return to showing the articles assigned to the category as it correctly did in step 2.

System information (as much as possible)

Joomla 3.9.4
PHP 5.6.39
MySQL 5.7.25