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But now a new problem: When I place an object on the ground and turn in a game mode that is only slightly turn right or left as a model disappears ... If we continue the rotation or to return retrospect it appears again .....
sorry your question confused me at first. this sounds like it could be 1 of 3 things.
1.) face normals: make sure all your face normals are pointing outwards before exporting from blender.
ctrl + n in edit mode should do this.
2.) your mesh: you might want to triangulate your mesh before exporting.
ctrl + t will triangulate your mesh in edit mode.
3.) your object's center: make sure your objects center is at the origin on x,y,z axis(0,0,0)
if it isn't and the player can step onto where the origin is, the map model will disappear in a way similar to how you described.
hope this solves your issues.