Got it working by deleting and recreating the backup job, and it ran fine for a few weeks...and then the dreaded TMP files have returned.  I'm the only one using this computer and this instance of QuickBooks, and am not changing anything.  I have no idea why the "fix" (such that it is) is not persistent, but it's disappointing to see that this issue has continued for YEARS.

If the engineers are truly "working around the clock to get this fixed" over the last three years, I hate to say it, but it may be time to swap out engineers, or provide them the resources to actually fix it.  I would upgrade to a new version of QuickBooks, but from all accounts, this unique "feature" of random disabling of automatic backups still exists.

I'll fix it again, and put a note on my calendar to check every 3-4 weeks so I can redo the scheduled job.  Talk about a Rube Goldberg workaround...