That's more or less what happened, if the digital displays polariser was damaged should the sunglasses bring the numbers up again in the damaged areas?
It sounds like the lenses were doing what you would expect them to do, you should see some action on one of the edges of the clearest part when you rotate them. Probably not that then.
Possibly the ribbon cable bond, that's another part that can be permanently heat damaged.
As Culzean says, the actual liquid crystal matrix in the lcd can also be damaged by sun/heat. Not common, but can give the same fading symptoms on reversed backlit lcds (like the Jazz radio), and is unfortunately terminal.