Blog Page Changes When Selected in Settings Reading

I found what I think is a bug – or at minimum I’m not using WordPress & Blog PAGE the way they were intended.

I’m using WordPress 4.5.1 and the latest version of the HighEnd theme as a child.

When I use any theme template page for use as the main Blog page [Blog Articles], the page creation displays correctly without issue.

ISSUE
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After the above page creation is complete and then I go to Settings | Reading | Front Page Displays section | A static page

and select [Blog Article] as the Post page, the page layout is changed completely following some predefined default somewhere. If I don’t select [Blog Articles], then the page works normally. I also tried locating where this default Blog page configuration is located but I can’t find it. Where is it please?

Am I using WordPress incorrectly in that these two functions are not meant to work together or did I find a bug?