Sprites suddenly missing , is it a 4.3.2f bug?

I have this project with many 2d sprites. I just imported the images and voilà, they appeared. Now some sprites are not showing anymore. The warning about NPOT images not being compressed to PVRTC used to appear before, but now the 2d texture images are not sprites anymore, and there is a lot of "Missing Sprites" in the 2dSprite render components of the game objects. The only way to make them show again is to change them from Compressed to "16 bit" and reconnect them. I would like to know if it is a bug or if we are forced to work with POT images to make it work. Anyhow it feels like a suspicious non-consistent behavior... Thanks for your help