This isn’t a bug, but a common newbie user error that the Discord community helped me solve, and I thought others might want to know. 
I was editing my first project in the UEFN editor, and when I pressed the “Play” button at the top tab above the viewport, Fortnite opened up and my player was spawned into an empty grassy Fortnite Island, in the middle of water, instead of the blank grid island I was working on with the assets I was using.
Here’s the solution xor, RodneyHoule, and Axel shared with me in Discord: 
TLDR - try checking if you’re actually editing a level not a new project, and try changing the Default Map in GameFeatureData to the level that you want to spawn into.
Details:
First, I had not created a New Project like I thought I had - I had actually created a new Level within the default MyProject file that I had been presented with when I first opened UEFN. Under the File menu, I chose File > Create > New Level, when I should have chosen File > Fortnite > Create / Open Project.
This is why my level “BluesFirstUEFNGame” didn’t show up in my main project library or my FNC rift, it was a level within MyProject. You can see this in the MyProject Content Browser.
Second, I was being spawned into an empty island level called MyProject. BluesFirstUEFNGame needed to be set as the Default Map in GameFeatureData.
So ya, if you’re having this issue, try checking if you’re actually editing a level not a new project, and try changing the Default Map in GameFeatureData. Good luck! 
