I can find many separate bug reports/complaints about Glass having transparency and reflection-related issues, including the problem fixed in the OP. I linked two in my previous reply. Here is another one. Here is one where the problem fixed in the OP is the root cause of the issue. People have been reporting it.
While this may be true, this is again a consequence of developers relying on unintentional behavior, which is why developers should not do it. Roblox has no obligation to keep bugs in their engine, and this is not the first time Roblox has made a change to UB/deprecated features that break older games
My point was that relying on unintended behavior/bugs to achieve certain effects is a bad idea. Relying on the many quarks of the Glass material is a bad idea. The Roblox engineers know Glass is a poorly behaving material and trusting it to stay that way is asking for trouble.
Don’t know what that is but that’s asking for trouble, that will likely also be patched in the future as Roblox improves their rendering pipeline. Needless to say, this all points to the root cause of developers not having enough graphical control