I still don't understand, because there is no rule like that. There are two factors in play, which apply to materials in general, and not emitters specifically:
- Output > Face Materials; the options are: Use front or back, or Use front only.
- When no material is found, one may be inherited from a containing group or component.
As such, here is a case in which removing the material from the back of a face would cause it to emit light:
- Output > Face Materials is set to Use front or back.
- The face has no material on its front, and a non-emitter material on its back.
- The face is contained in a group/component that has an emitter material applied.
Removing the face's back material causes it to inherit the material of its parent entity. Is this what happened?