To get this to repro 100% of the time a new Blueprint 2D Side Scroller template project must be used.
If you close the project, then reopen it the project opens to the correct map. I'm assuming it's saving the fact that I chose to open the default map from the previous attempt.
This does NOT occur on the same template/project type on Windows or Linux.
I asked our Engine Content Contractor about it and he said the project map was set correctly.
The project launches with a black screen and a message box about being unable to open a certain map and if I want to open the default instead.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-34071 in the post.