Molten glass will exchange heat with tiles below the glass forge before it gets placed in the output pipe. This happens in vacuum too.
This can lead to glass solidifying in insulated abyssalite tile when there isn't any backup.
Changing the surface below forge to insulated abyssalite tiles will prevent that.
1. Build an empty (vacuum) granite room with a water lock 2. Place a glass forge inside, with insulated abyssalite tile output that leads to vent outside room 3. Craft glass 4. Notice that most of the time, some of the glass solidifies as soon as it is placed inside the pipe 5. Change room material to insulated abyssalite 6. Craft glass 7. Glass no longer solidifies