opacity mask does not work

Development Rendering
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1

hello, there.

I am trying to make a vanishing material as reffered this video: Creating a Vanishing Effect with Advanced Materials | Feature Highlight | Unreal Engine Livestream - YouTube

But on preview in material editor, opacity mask does not work.
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as you see the attached screen shot, I selected ‘masked’ for blend mode, then made opacity mask value 0.333

then there should be nothing on preview, but the sphere on preview did not disappear.

2

I found that when ask preview by selecting ‘setMaterialAttributes’ Node, opacity mask doesn’t affect the mesh’s visualization.
I don’t know reason.