[londons_explore]

My guess is the LED's suffer reverse bias thermal runaway when they're hot from being in a steamy enclosure and then they get a reverse 5v across them and any leakage current turns into heat accelerating the process.

[colechristensen]

All LEDs are photodiodes too, certain degredations of parts or poor circuit design could lead to the display turning into a switch.

[CGMthrowaway]

Wouldn't it be more likely to be reverse-bias degradation of the LED junction causing permanently increased leakage current?