I've been able to see this problem just once, right after launching PI, opening an image, opening the HT tool, and clicking the RTP button. The image was shown by RTP but then it disappeared just as you've described.
The bad news is that I'm being unable to reproduce the bug anymore, and I've tried!
How frequently do you get this problem? It seems like a really spurious error. I really hate when something is so slippery!
Thanks for detecting this glitch.