So I found it! The ball on the top right has the "Affected by Corona VFB tone mapping" unchecked, which means that the inverse tone-mapping operator is applied to the output shader. This is useful if you want your textures to be unaffected by tone mapping - e.g. for an environment map. But if you use the output shader in a map slot of something which does not get influenced by the tone-mapping (bump map, normal map, displacement map, glossiness map, IOR map, etc... basically any map that does not result in colors being rendered on the screen), only the inverse operation is applied and this results into the white being changed into grey...
Unfortunately, this is not shown in the shader preview, so you actually see white output and think everything is OK. Changing the preview to work consistently would not be so simple, so for now, I think we will add another sentence to the warning in the output shader.
In any way, thanks for catching this!