For people who are having the problem falling using level streaming, If you use an open level node and the “level” is a level streaming level, the problem is that there is no object for the Character to stand on in the persistent level once spawned into the persistent level. So an easy fix without all the code and gravity options, duplicate the floor/plane that you are using in your first level. Click on the persistent level and paste the ground in that level. go to details and go to collision >Collision presets>choose invisible wall.
Then scroll down to rendering and uncheck the visible .
This fixed the problem and a lot of what was explained fixes was not working for me. This did, also there will be a moment where the persistent level is the only thing loaded (less than a second), so make sure you have what you see on load in the persistent level and destroy after .2 delay when its loaded or use a widget overlay with a start button. That is a quick fix there are other methods but it wasnt working for me. just saw alot of people had this problem and i just got through this issue. hope it helps