I have seen one other person who had a similar issue, but I was not able to resolve it from the answers there. Basically when I launch the game I immediately fall through the floor, but I can play in editor just fine. My other projects work fine on launch. I read this might be because the first map you make in any project becomes “persistent”, and every other map is a sub level? So my projects that are working only have one map, but on my “problem” project I was trying to start on the second map. I have looked at trying to make that map persistent, but no luck. Anyone have any suggestions or clarification? I even deleted the first map, but it’s still having the same problem. Someone mentioned freezing your character (no gravity, no input) until the map has fully loaded. I may start googling how to do that but I am hoping for a little more info about the situation. Thanks!
I am haveing this same problem
If anyone else is having this problem, try enabling the Should Block on Load option in your Load Stream Level nodes.
I did this. My character still falls through the map for some weird reason
Fixed it for me. Thank you 
Also fixed it for me. The level that was loading where my character was supposed to land was not persistent. I was previously relying on automatic loading since I am using world composition. But I added the Load Stream Level node in my persistent level’s blueprint and enabled Should Block on Load and now the character no longer falls through in launch. Thanks!