Last modified: 2009-12-30 08:51:16 UTC
When you attempt to sort a query by a N-Ary property, only the value of the first part of the N-Ary property is used. The other values are simply thrown away. The buttons at the top of the columns work, though. The URL is to a test case on the sandbox wiki.
In fact, there is no sorting support based on n-ary properties at all. If the items appear to be sorted in your example, then this is a coincidence. The implementation of this feature requires changes in SMW's SQL query creation and execution.