I just noticed my display does exactly the same thing. It just says "Coldest day this week" with no temperature indicated.
I don't know what the display interprets "this week" to be. Is it the last 7 days, or is it a 'calendar week'? To be honest with you, I've had my displays for over a year and I have never looked at that corner of the display to notice what goes on there. Like many others, I wish the display was much more able to be customized.