Melt-out is common on white-centred leaves. Best preventive is to plant in more shade, but then many hostas like some sun. This year, if you've had the kind of weather we had (horrible wet, cold spring followed by very hot and very dry weather), we're seeing more of that problem. I grow 'Morning Light' on the east side of my hose for that reason and most years it does OK, but this is what it looked like last week.
Yours MIGHT do better without the afternoon sun, but would probably like some morning sun.
