Steps to reproduce the issue

First test

• Add category blog and sub category sub-blog.
• Add articles in each category e.g. with alias blog-article-01, sub-blog-article-01
• Add menu item for category blog blog. Set to default page to make it the home page.

Second test

• Add menu item for the sub category below category blog.

Expected result

Both tests

• URLs for Home
  example.com/
• URL for blog article blog-article-01
  example.com/blog-article-01
• URL for sub blog article sub-blog-article-01
  example.com/sub-blog/sub-blog-article-01

Actual result

First Test as expected
example.com/
example.com/blog-article-01
example.com/sub-blog/sub-blog-article-01

Second Test
example.com/
example.com/blog-article-01
example.com/blog/sub-blog/sub-blog-article-01

Additional comments

example.com/sub-blog/sub-blog-article-01
vs.
example.com/blog/sub-blog/sub-blog-article-01

This happens if you add a menu item for the sub category.
The links should be the same in both tests.

Two URLs - same page
example.com/ and
example.com/blog
show the same content. The alias blog should never appear anywhere.