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experiencing issue where emitters behind emitters with opacity mapped either to layer mask or transmittance channel are visible when assigned as hidden.

this probably illustrates it best....





thought it could have been due to being inside concave mesh, but happening anywhere and everywhere...

My take:
Hidden to camera on an emitter only affects the direct light from the emitter to the camera. Once it passes through a material, even if it is completely transparent, it's considered bounced light.