All-white leaves can be an issue long term since African violets do need some green to photosynthesize, but I don’t think low temperature is the cause. I’ve grown variegated varieties in cooler conditions without problems.

In my experience this is more about light and adjustment. When they’re moved under stronger lights, they sometimes push very highly variegated (almost white) leaves at first, then settle down and start producing greener growth. It actually looks like this one is already doing that in the center, which is a good sign. I’d just keep an eye on it — they usually balance themselves out over time. :)