Yeah thats what I thought should happen, turns out that this bug was with a UI that shows at the start of the game, (not the end of the round - I just tested that and it does work).
I did some checking, and it seems as though ControlScript loads into the player after PlayerGui elements do, and therefore jump was actually bound over the top of my advance button.
I just added a WaitForChild(“ControlScript”) and the problem was solved.