Things like this can happen in brittle materials. There is a daily thermal cycle, and depending if the glass panel had a defect, or it was put in the holder and stressed too much. So going from cold to hot every day, even 30-40 degree temp differences, makes a big repeated stress on the item.
And finally it will fail catastrophically.
Not much to be done about it, get another bit of smoked glass and put it in the frame. Or order a new on from WB.
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2020 Winnebago Vista 29V
San Francisco Bay Area
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