Basically the game isnt recognizing its a window. It happens with open door frames or regular doors too. The doors are functional but in game the windows look like walls with a window frame like ...
Ideas
It's been doing that for years. Go somewhere like Newcrest and build a bunch of houses for a community you want to move in there together, and you'll see bricked-up windows and doors all over the place. If you select the offending window or door it will fix itself, but half the time another one will glitch out straight away. It's especially annoying if you don't notice it until you've moved on to the next property, and you look back to make sure you're matching the style of the neighbourhood and realise the house you just finished has a bricked-up window.
