If you do volume interface for clear acrylic, then the black acrylic is not visible to camera, it may look like it is, but actually it is not because of specular–diffuse–specular (SDS) paths are infinity times slower.
If you add 3'rd interface the 1 second render has infinity times more specular reflections than 1000 hour render.
Handling of relative refractive indices
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Please upload a testscene that allows developers to reproduce the problem, and attach some images.
Please upload a testscene that allows developers to reproduce the problem, and attach some images.