Unity 2017.3 Timeline hides other UI element;

Hi, I am facing a weird problem, my timeline is basically a canvas and a few UI images in that canvas.
When introducing Timeline into the scene, it hides out other UI elements, even if the element is at front in the scene hierarchy.
For example I have an UI Image which says "tap to skip", it can't been seen no matter how you place it.
Weird is, if I have a button, it is still interactable even not seen.

 

it seems if I place the UI element on the same canvas as Timeline elements, the UI image can be displayed;
But if I place them on a separate canvas, it is hidden..