It is because you use the Denoiser, it blurs transparent shaders.
Unfortunately this issue won't be fixed, in fact it is more a limitation than a bug...
Here is an explanation from Thomas, one of the developer at Abvent in charge of the rendering:
The algorithm doesn't know if the material has a reflected, a transparent or just a diffuse channel.
When the energy/light variation is low on a surface, it blurs the result, whatever we could see behind the glass, or through the mirror.
It is a limitation inherent to the chosen algorithm.
In other words, the algorithm works only on the first shading level.