If you use an open level node and the level is a level streaming level, the problem is that there is no objects 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
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