Well, when one makes glass in the glass forge, the produced molten glass already cools down before completely leaving the building. In case one has something like water there, the glass (usually the last 5kg package but can be earlier) will break the first pipe regardless of the material used for the pipe.
This can be easily* fixed by preventing the building's content to exchange heat with the outside i.e insulating it.
*if it is actually implemented/defined for each building separately instead of an inherent property of all. Making an exception may prove more difficult, perhaps.
I hope you can fix it and consider it a bug in the first place, since it sure could confuse people, or already did (I already searched but no bug report as far as I could see yet).
Save, just to easily see for oneself (incidentally dupes choose a new task when loading a save plus glass making progress is reset, that is the closest I could get)
Screen, for what's it worth (see the molten glass in the forge, the 5kg is well below the freezing point of 1126.9 °C at 768.5°C and when it is normally produced at 1741.9°C):
1: Build Glass Forge and fill the bottom tiles with water --- 2: Make Glass --- 3: Watch the Contents cooling down and watch the first pipe break