So paddling around further, I found that not all my zones exhibit the problem, which led me to try a bunch of different setups. I eventually narrowed it down to some kind of issue with using my soundcard (a Steinberg UR824) in ASIO mode. All the zones that were exhibiting the issue were using the Stenberg in ASIO mode, and if I copy one of the misbehaving zones and change the sound device to a different device (or to the Steinberg in WASAPI mode), I can no longer reproduce the issue.
The problem, of course, is that I can't access all eight channels of output on the Steinberg except in ASIO mode and, of course, everything has worked perfectly for years up until now. I tried uninstalling and reinstalling the Steinberg drivers, with no change in behavior.
I generated a log that exhibited the crash, FWIW, in case there are any clues there. There definitely appear to be some directx/madVR errors and a number of audio errors, but I'm not sure how significant they are.