In the past, BST's have overwintered in my house in a downstairs room without a problem and eclosed some time in May.
Last year I made the mistake of leaving the cage with one last BST chrysalis in an upstairs room where the sun pours in the window. We had some late Monarchs so were trying to keep them warm to hurry them along, and had moved our butterfly operation there. Well, it didn't occur to me that the WINTER sun REALLY pours in the window. The poor fellow eclosed in February -- only lived for a week or so.
. (I'm glad there was only one.)
I believe the difference is that in the downstairs room, the cage had been kept against the window wall so that no direct sun ever reached it. It's colder against the wall, and generally colder by 3 or 4 degrees between upstairs and downstairs too. If you do some sleuthing around the house with a thermometer, you might find significant temperature differences too.