PNG thumbnails sometimes black sometimes white?

When I save out transparent PNGs, the thumbnails usually show white where the transparency is, but every so often the thumbnails show black instead of white.  The file itself is fine, with transparency intact. It's just the thumbnail that is wrong. I'm not saving the files any differently from one another, just using Save As and choosing PNG. Does anyone know what is causing this of how to avoid it?  I'm asking because I've had clients complain.  It must be their viewer app is showing black so they think it isn't transparent, even though it is.  It happens often enough that it's tedious time-consuming to have to re-save them in order to get a white thumbnail.  Any suggestions appreciated!  Thank you for your help!