The bug is more often than not, but doesn't occur 100% of the time. After posting below, I left the two blank canvases on the wall and traveled away and then back - to see if the images would load once the lot reloaded - but they did not. I then traveled my sim to the beach and had her do another paint by reference. As you can see in the screenshots, the painting never appeared on the canvas as she was painting, but the image was there in her queue when I queued her to put the painting in her inventory (the first image - lower left corner), and the image was also there when the painting was in her inventory and also when she got home and hung it on the wall. This one was also a masterpiece so the quality of the painting does not seem to be the trigger. I'll try to start paying attention the the sim's moodlets and situation/location when painting to see if there is a common trigger when the paintings remain blank.
