Searching through tons of topics and pictures but still confused as to what is affect the plant.
We thought it was just regular dying leaves, but the way the leaves are shriveling up looked suspect to me and one of our begonias just died from whatever it is.
Both new growth and old are affected.

I don't see any fuzzies. We did water them with nematodes recently to get rid of fungus gnats, and our other begonias in the other room seem fine. They've been in the same window where they've flourished, but maybe one of them is getting less sun as usual?