Noel, this was actually super helpful for me in working around a different issue! I was super excited to get Task Queues for exporting bounces, as I have an entire record in one session file and so being able to set up a variety of time ranges + export settings is really ideal.
I was finding that certain songs would write a small placeholder file at the beginning of the task, and would appear to export the audio, but then would never write the final file. The issue was reproducible to an extent but would occasionally not happen when I exported those tracks subsequent times.
After reading your comments about null busses, I remembered that I have four additional channels which are not assigned to any output bus because they're just holding lanes for editing purposes. The four songs which exhibited the issue consistently were the ones which had wave data in these channels.
Cakewalk complains about these channels every time I open the session - I can't say I wasn't warned! - but until I started using the Task Queue it had never been a problem to leave their outputs assigned to nothing. I have those channels muted anyway so I just assigned them to the main buss and it totally resolved the export issue; all the files are being reliably written every time now. Thanks for your insight, Noel!
