Whenever a CanvasGroup is made visually visible for the very first time, all of its descendant UI elements flicker for a split second. This behavior isn’t tied specifically to the .Visible property; for example, it also occurs if the CanvasGroup is a page within a UIPageLayout that is being shown for the first time.
This flicker only happens on the initial render. If I hide the CanvasGroup and show it again, the transition is perfectly smooth.
My theory is that this is a rendering optimization. Roblox doesn’t render contents of UI elements that are not visible. When the CanvasGroup is made visible for the first time, the engine has to perform an initial render of all its contents, which causes the noticeable flicker.
Video of it occuring: