The problem is that when the VirtualTexture lightmap is enabled, the value returned by the material node's PrecomputedAOMask is incorrect.
1. Open an attached project.
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2. The static meshes for this project are set to display PrecomputedAOMask.
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However, it is not displayed because the virtual texture lightmap is enabled
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3. When Virtual texture lightmap is disabled and light build is executed, PrecomputedAOMask is displayed correctly.
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There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-84986 in the post.