Still more research results:
The extra personal calendar doesn't get created when you add a class, but when you add a recurring event to its calendar.
I tested this adding first a real-world class to the system and then adding its lectures to its calendar.
Note: This probably has nothing to do with the bug, but all the events were in the past, i.e. from September 6th to October 20th, because I really copied all the facts from the real course :)
