No, I don't think this is a bug. As I mentioned, this rarely happened with my NI Komplete Audio interface. It has only started happening since I changed over to my Sapphire 6i6. I understand the deal about overlapping notes but, this was never much of an issue previously. Still, as chuckeybaby suggests, might be worth cleaning tracks up with quantize. Bit of pain, though. 

well ok then

Ruckman65
might be worth cleaning tracks up with quantize. Bit of pain, though. 

not as bad as you think man.
- Select the whole track then press "Q" to bring up the quantize menu.
- Set quantize to just barely pick up note durations. Click enter.
- If you find the notes are shifted too much, easily hit undo and try another quantize duration setting.
 
You don't really need to do any hard quantizing here, just simply making sure you don't have any overlapping going on. But you may want to do a few tests:
 
- Listen for the area of stuck notes, are they in the same places (repetitive) every time ?
- Open the PRV and check the notes to see if they are overlapping.
 
Even if its not an overlapping issue, this will help you narrow things down a bit.

If it's really only your Sound Canvas, it somewhat points to MIDI interface, cable, or the SC itself, no?

If it's only with hardware synths, and only with one hardware synth (that we know of), I'm inclined to agree it;s probably a hardware/firmware/driver issue. I use a number of hardware synths, and have pretty much never experienced a stuck note during playback. But notes may 'stick' on stop if the track has CC64 Sustain messages in it and 'Zero Controllers when Play Stops' isn't enabled in MIDI preferences.
 
SONAR sends CC123 All Notes Off as well as individual note offs for sounding notes when play stops, so that should silence everything except the effect of Sustain or Sostenuto which override note off messages by design and patches that have a long release stage or long one-shot samples.
 
If overlapping notes of the same note number are causing the problem, I would still suspect that's a hardware/firmware/driver problem. I wrote a CAL that eliminates overlaps to eliminate the effect they have on the sound, but I never encountered any stuck notes before they were removed.
 
P.M. me your e-mail addr, and I'll send you a copy of the CAL.

As the Sound Canvas has been working flawlessly for years, I reckon I can count that out. I think it all points to the Focusrite drivers though I am not at all sure how a driver would effect midi data that way. I will try your suggestion, chuckebaby, and see how that goes. I have pm'd you, brundlefly, and thank you so much for your generous offer. 

I tried an experiment. I copied one of the midi tracks with the stuck notes and pasted it into a new project. When I played it back in the new project, no stuck notes. I checked as many settings as I could think of in the old track and compared it with the settings in the new tracks but they appear to be the same. Maybe the track in isolation doesn't stretch the midi capabilities of my setup. Though as I mentioned, this problem only started after changing my audio interface to the 6i6.