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When using transparent objects in front of the Billboard, make sure to follow the steps described in the "Objects in Front of the Billboard" section here: https://aximmetry.com/learn/virtual-production-workflow/green-screen-production/unreal-scene-setup-green-scre#objects-in-front-of-the-billboard 

If the object is translucent, also follow the instructions under "If the object is translucent, the object's material also needs to be changed." Also, review the "Translucent Objects at the Billboard" section.

Warmest regards,