Additional news. I talked with a Microsoft Insider and it seems that the Jet Relational Database Engine used by Scheduler is alive and well in Microsoft! It is being used in the support infrastructure for the Microsoft HoloLens 2 project, other areas such as Azure, and of course within "billions and billions" of Microsoft Access based applications. It's reassuring to know it is highly regarded and in widespread use outside of Microsoft Office.

Since we haven't seen a fix come out, it's possible that I am using some combination of features that, taken together, were broken by Microsoft for database size stability in the last update of Jet, and thus not affecting everyone. I naively thought "oh we'll just diff our code and knowing what the problem is, we should be able to see where we screwed up". It appears that may not happen (still holding out for a fix from them), so I am going to put together the simplest possible repro scenario I can. It could be a big job because there are so many moving parts in the database, the stored procedures, the SQL statements and the logic in my scheduling object model wrapper layer on top of the database API.

Stay tuned, I'm on it.