Vinyl going bad is what makes them look like that.
Only way to make them last is to paint second surface on clear polycarb then spray translucent white flood coat. Learned that from an old time sign guy I worked with from time to time. He did a lot of backlit work, some of which is still in my neighborhood over 20 years later.
The rare backlit panel I need gets subbed out now, but it's still second surface on clear with white flood.![]()