Trouble with UI Mask

Hi guys - first time using UI Masks. I've setup a Mask as a child of my Canvas in world space. The children of the Mask are RawImages which I slide through the mask. When I run the game, none of the children render at all. If I go in the hierarchy view and toggle the "Show Mask Graphic" checkbox on the Mask component, it starts working and continues working after that point.

Toggling the same variable from c# doesn't seem to have the same result; none of the children render until I manually toggle that value from the editor.

Are there particular material requirements for the mask or the children? I'm using a plain material with the Default UI shader type, but had the same results when specifying no material at all.

Is there any special "sync" function I can call on the mask to force it to re-render as it does when I tweak its values from the editor?

Thanks for any insight.