Ever since the introduction of the Glass material, there has been a bug where transparent parts behind glass parts would be occluded. This trick has been very well known by the community and was also acknowledged by Roblox engineers at the time as “unintended behavior they would keep because of its usefulness”.
However, recent changes (sometime around December 2020) to rendering have resulted in this effect no longer working on graphics 1 and 2, and for graphics 3+, the effect would only work up till a certain distance from the glass part.
Here is what you see on graphics 1, notice how the part behind the glass is not occluded. (glass part in front, 0.1 transparency part behind):
Here is what you see on graphics 3, notice how the part is occluded (this is the behavior that Roblox devs like and rely on to pull off certain visual effects which would otherwise be impossible):
Also, here is a demonstration of how the glass rendering trick stops working after a certain distance: https://gyazo.com/356eef3b177726ea309878bb85569e38
Reproduction:
Place any transparent glass block in front of a semi-transparent smoothplastic part. Observe differences in occlusion behavior. You can also see this here: glass trick - Roblox