Thanks for all the tips and patience! It’s a small map, so I do not have world composition enabled (but i’ll try it next).
I’ve gotten the force load all cleared out (so i’m back to the default world settings except for editor enabled NPC’s), I’ve put the map name “UberMiniLevel” in the map section. Still flipping back to the menu upon loading 
If you still are having problems just put this map on hold, take a look at either brick’s series of video tutorials or see if a dev’s short tutorial video is enough to start a new map that works from scratch. Then once you get that working maybe you can come back to this map, give it another look over at what may be causing a problem.
Did you ever find a resolution to this issue? I am currently experiencing the exact same issue. I’ve replicated all of the steps on both tutorials listed above, and copied settings from uzumi (love your work btw) and still cannot get my map to load in single player for testing. It will load perfectly fine when in the editor (even on the standalone selection). But once cooked and uploaded to steam, I get tossed back to main menu on the client whenever i try to load it…
If you found out what your issue was, can you please let me know as it has been driving me up the wall! Sorry for being the post necro 
So for anyone else who has experienced this issue. Let me be very clear about the issue I was speaking about above. My map worked fine when playing through the editor or launching the standalone game through the editor. Whenever I cooked and uploaded to steam and then downloaded on the client and tried to load the map it would just kick me out to the main menu. After trying a few different times I ended up deleting my workshop file, renaming the map through the editor, and re-uploading. Literally leaving everything exactly the same aside from the name change, I recooked the map and uploaded it back to steam. Subscribed to this new workshop file, launched my game, loaded it up, and I was in. I have no idea why this happened, what caused the original map not to work (perhaps it didn’t cook or upload correctly) but this solved the issue for me.