I experimented with the grouping and leaving the thickness alone, and this is my result. It does look alot like yours...
But then I noticed you had the Sunlight button ticked. I think I remember db4tech saying that the compute glass transparency won't have an effect if the sunlight button is ticked, so I unticked it, and moved the group of blocks to just pass the inner surface of the wall. I made the blocks slightly thicker so that no surface could conflict with each other, and this is my second result....
I think most of the trouble is coming from the surface of the wall face, and the block faces being directionally on the same plane. The engine has to decide which face is "dominant", and where the light will pass through. Of course, db4tech and puybaret will likely know more about this, and my attempts are only guesses.
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