Thank you @pichlo, your explanation might be correct!

I am aware of the 1st fact you describe, I too played with watch LCD displays and batteries when I was a kid and remember that the number segments only blackened for a moment when DC voltage was applied over the electrodes.

However I never came across the 2nd phenomenon that you describe, that there was permanent change in the display when the charge was left on longer. That indeed seems to be what's happening here.