Why do some elements of my level disappear when i play?

I’ve got the following problem. There are 2 levels in my game, the main menu and the world. Whenever I start playing my game from the Main Menu, once I change to the World some elements disappear and my character spawns in another position. It’s like the engine does not save the elements I have kept adding to the level since I created the menu, but if I close the engine and open it again those elements that had disappeared are there again. Can someone help me?

do you have a video? quite hard to understand. if you mean your in level menu and add things to the worl like spawn objects by code and then you change the level to a new world - it s normal these things from menu are gone