Yes to all questions. The GUIDs are there on export, they are there on import (because they come in correctly when I create the table fields manually), and it does appear to be repeatable. If a survey has a problem one time, it will have that problem every time. If a survey has no problem...well, I guess I can't say that it won't later have a problem. There is no difference in how good and bad GUIDs are created, nor is there any obvious difference between them.

However, as I was writing that, I did think of some other things to check, though they are unlikely to matter. XML is fundamentally a string. Perhaps there's a difference between the GUIDs that work and those that do not work, such as, perhaps one starts with a digit, and the other starts with a letter.? I'll have to look for differences like that.