However not all foliage have this Clouded Green Features - as this appears only in 1-2 leaves in the whole plant, sort of the begonia is pushing off the variegation away from it's system as some plants do when they try to revert to their origin parent plant.
However, not all begonias are hardy and able to morph to such conditions - Begonia Iron Cross are not so forgiving - the dark pigmentation slowly disappearing into a thin line with the edges appearing burned and eventually succumbs to its death if they are overwatered or lacking humidity. These strong furry types requires special conditions but some can be acclimatized when the right conditions are met:
Water only when the Medium is Dry
I use Foliar Fertilizer - Once a Week with 1/2 Dilution Strength.
Begonia Common Names and Images