Probably the level is streaming in runtime. If by chance level streaming is delayed for a bit the character will fall down.
- I suggest delete the ThirdPersonInstance and again drag the ThirdPersonMap into your map.
- Remove Player Start From the third Person map.
- Manually place The player start in your map.
- go to world partition and load the selected region to see the instance in editor (Only if the dragged ThirdPersonMap Instance) doesn’t show up in the editor