The code you have here shouldn’t be deleting them, so I would consider checking the properties of each sphere to make sure that they are anchored and collisions are enabled. If you have any other scripts running, you might want to look over them to confirm that they aren’t altering the spheres.
This isn’t the issue with your code, but please don’t do this. You can use TweenService for smoother and less ugly (code-wise) results. Also, game.workspace is deprecated. You can do game:GetService("Workspace") (get any service based on its classname), game.Workspace (use the property of game that only exists for workspace), or workspace (a global that refers to the workspace).