Update (short version): The patch *appears* to work. I say *appears* because I want to do a little more messing around, but so far it performs as advertised.
Update (long version): At the start I no longer had a saved version of the "problem" project. Since the envelope problem first arose I had made numerous modifications to the project in order to work around the envelope issue - most particularly removing the SIR reverb and using clip envelopes rather than Track envelopes. So I had to try and recreate the project in order to see if I could duplicate my original problem.
I also, since the time I had the envelope problem, had lowered my Sonar latency settings to about 5.8ms.
After attempting to recreate the project, I hit the play button. While everything appeared to perform as normal - no sound came out. I rechecked everything, and it all looked OK.

So, I move the latency slider and raised my latency up to about 75 ms. Once I did this, the project played normally, and bingo the envelope problem raised its head again. HOWEVER, I then lowered the latency down to about 40 ms. The project still played fine at this setting, BUT the envelope problem went away.
Increased latency back to 75 ms - and the envelope problem reappeared. Lowered it - gone. Raised it - back.

WTF??
All of this, btw, is BEFORE installing the patch.
Anyway, I went ahead and installed the patch. Now I could no longer get the envelope problem to reappear, regardless of my latency setting.
All of this is a bit puzzling to me, which is why I said earlier that I want to play around a bit more. However, the good news is I could not recreate the envelope problem once the patch had been installed.
Stay tuned.