Many years ago, mine had leaves like these but it seems they're all solid green now. I'd like them to come back but not if it's a stress response. Or is it just something only younger trees do?
I think it has something to do with how much light the tree gets. I just got into house plants this year but I noticed my dark leaved rubber tree becoming more green so I moved it to a brighter spot and now its turning darker again. Maybe yours wants more light as well?
With winter (and shorter days) coming, I've invested in a few full spectrum grow lights that I turn on in the morning/evening to keep the variegation spicy.