I'll contact Support, but it's complicated enough that I want to do more work first to see if I can narrow it down.

Just as a random thought for debugging this, check to see in the first case if the symbols defined in the package also exist in the Global` context. This may lead to nothing....

I was concerned about that because some of the symbols were Private symbols in the package. But then we carefully picked symbols that were not Private or exported. The main symptom of failure is that the notebook loses connection to the package context. And this even occurs on subsequent Input cells where exported symbols get changed to symbols with the package context attached - because it no longer recognizes it.