It's not a bug. That's how it's supposed to work. The ceiling reflects relatively dark ground color of CoronaSky and bright color of the curtain. Curtain scatters light in all directions, including towards the ceiling, where as glass does not scatter any light, and just passes through color of the CoronaSky ground. If you place your camera at that exact place where the reflection occurs, and point that camera down towards window, you will see what kind of lighting arrives there, and you will see that from the point of reflective ceiling, curtain indeed appears a lot brighter.