It's not uncommon for the sectorial white variegation of variegated aroids to brown quickly, especially when the plants are stressed. Remember, white plant tissue has no chlorophyll. It does not photosynthesize. (neither does the all pink tissue of Pink Princess Philodrndron). So it will naturally die faster than green tissue, and patches of white (sectorial variegation) decay faster than integrated splash and streak variegation.
There really isn't a fix. Your white parts may stay longer after your plant stabilizes and starts growing if you provide adequate light and humidity
