Thanks for the incredibly fun demo!
To replicate the bug, after the first scene, i remember looking for all 5 marbles and entered the mechanical arms room again.
Needs to be clearer where to obtain Marbles for the quest. Maybe add more "hints" for location.Quest status gets bugged when I progressed and begin the cutscene for mechanical arms a second time.Maybe also add a skip to cutscenes if you already watched once.