Hard to say w/out a pic, could you pls show us?

Diagnosing blind like this, I'd guess it's a drainage problem leading to dehydration.

From the few Hoyas I've seen left in too much sun, too much sun just bleaches out the foliage (makes pale in color, light green to almost white.)

Is it an EA plant perhaps that was left in its original mix? Tho' I can't say I recall ever seeing EA selling H Pubicalyx.