I think the issue stems from you altering something in the StarterGui model instead of the Player’s specific Gui.
You can do this by:
local loadingUIcube = game.Players.LocalPlayer.PlayerGui.adminPanel.LoadingScreen.loadingTheUI.cube
This will access the UI that the player is currently look at. You should consult this article to better get familiar with the StarterGui folder and how it actually copies objects to the PlayerGui folder for each player when they join the game and when they respawn. StarterGui (roblox.com)
Make sure this script is running client side then add this line, local playerGui = game:GetService("Players").LocalPlayer.PlayerGui
then you can just call playerGui for this particular player’s Gui.