Afraid that you are correct. When we first got a 622, "trick play" worked great. Then Dish had the problems with Tivo, and ever since then it has been impossible to stop the action and then step through frame by frame. The picture is liable to jump anywhere. This has been the case on a couple of 622's, our first 722, and now a replacement 722. And it is not the case that the receiver has to jump to a "key frame". No way that "key frames" are 15 minutes, or more, apart.
You're right about single-frame skip mode...this is a different problem that using skip during normal speed play back.
If you pause the recording, then use the skip back or skip forward expecting to skip a single frame at a time you will get unpredictable results..skip back is worse than skip forward a single frame...
Frame mode skip forward works pretty well if you continue to press skip forward to get frame-by-frame skips..but pause between skips and the next skip may take you many seconds forward..
Frame mode skip-back usually only works for one or two frames back, then totally unpredictable...pause for a few seconds between frame-skip back commands and you may even skip back to the beginning of the program..
These problems don't show up using skip-forward or skip-back in normal playback speed...IF you pause slightly between skips..skip-forward is a reliable 30 seconds and skip-back is a reliable 10 seconds...but must have a slight pause between skip commands..
The pause-skip-forward or pause-skip-back problems have been in my 622 since day 1...5 or 6 years with my original 622...one of the earliest manufactured and issued..