The Special Teaching actually occurs on the last Sunday of each month, but google’s recurring event scheduler doesn’t support “last.” That’s why we use the 4th Sunday as the recurring day.
There were 5 Sundays in July, so we changed the date of that occurrence from 7/22 to 7/29. I guess that’s the source of the problem, but I can’t follow why the ICS spec wouldn’t address that as expected.
If I understand your explanation, there should still be a recurring event instance 20120722T110000 that shows a date modification to occur on 7/29. You say that there is no such instance – but that appears to me to be a bug either in the spec or in google’s implementation.
That same instance could then have its description modified without creating an additional event.
