Plant help

Hello all! I can't figure out why this Begonia has leaves that get weirdly weak and then eventually fall off. It's fairly humid where I live (south east Qld, Aus) with the average humidity being anywhere from 60-90% most days It only seems to be the leaves on the taller cane of the plant that are having issues. I've added some photos 😊 Top of the soil looks dry, but it's still moist 1 inch deep