This is a fundamental problem of screen space effects. Some parts of the window want to refract a part of the sky that’s covered by the bars of your window. We have a similar problem for water reflections, where we want to reflect something that is blocked on screen by another foreground object. This thread has more info: Reflections in water cut off by character.
We’re looking at ways to improve this, but this is a hard problem to solve. In the mean time, I suggest keeping this artifact in mind and building around it if you’re going to use Glass for windows. From what I’ve seen, it doesn’t look too bad on large windows that the player never gets too close to or looks at from a sharp angle.
