Agree that moisture is likely the enemy here.

When I was in grad school, I fixed several electronic devices by placing them in a vacuum oven and using hard vacuum and moderately low (120° F) overnight. BMW used to (1998 740iL, for example) use a speedometer panel that had LEDs that degraded, and I think that the services out there that offered to fix them used the vacuum oven method.

Not everyone has access to a vacuum oven. Other desiccants are available that work a bit better than rice. Damp rid is calcium chloride. You can buy lime (calcium oxide) or the blue stuff (calcium sulphate, with a color indicator, aka Drierite) that turns red when it's wet. I think to reclaim your unit, I'd buy one of the little bags (ifixit's "Thirsty Bag"). Here's an article about how to get best results.

Probably worth a try.