Disappearing PNG textures with transparency

I already sent some screenshots and a description in to Support, but wanted to see if anyone else has had a similar experience. I'm using PNGs with transparency for textures, but find that they disappear occasionally, sometimes individually, sometimes en masse. When they disappear, the object appears like the default material. In the attached image, the two large vertical rectangles in the foreground were cutout people, then they weren't. After deleting and recreating the textures a couple of times, they eventually remained as textures. I eventually found them to be more stable when they were created from an existing texture, rather than having Default selected and then making a new material.