Are you changing the gui in StarterGui? If so, you’ll have to change the gui that is loaded in for all players by looping through each player. Their StarterGui would be in Player.PlayerGui
I think it’s a bug, I tried this in the command bar in studio and it changes the transparency, it just doesn’t visually change for some reason until it’s manually set.
I Could Just Not Be Reading The Situation Right But I Think The Issue Stems From The Fact That Its A Server Script And Anything GUI Related Needs To Be Local Side Unless Its Sending Messages Back To The Server