It was no longer possible to sort on the values of linked fields, except the key/foreign key field.
This is documented in http://demo.tiki.org in both Tiki 6.x ( http://demo.tiki.org/6x/ ) where everything works(!) and trunk ( http://demo.tiki.org/trunk/ ) where it is not working, and there was not solution prior to 17 April 2013.
Each demo version has two trackers:
1. City (trackerId=2 in trunk; trackerId=18 in 6x), containing the fields, City, Province/State/Region, Prov/St/Reg (acronym), and Country.
2. University (trackerId=1 in trunk; trackerId=19 in 6x), containing the Name of universities,
as well as Location (which is an Item Link field, linking to the City tracker, and provides also Prov/St/Reg and Country,
fields the values for which should be (and are being) retrieved from the City tracker
via the Key/Foreign Key (Name in the City tracker = Location in the University tracker).
This allows universities to be entered into the table simply by adding their names and their location (city),
with the rest of the information provided from the linked tracker.
Here in demo this is three fields, the province, the province's acronym, and the country, but in practice it could be dozens of fields.

In earlier Tiki version (see http://demo.tiki.org/6x/ for example) it is possible to correctly sort the University table by any field, i.e. also be Province, Prov, or Country. In trunk this no longer works.

Nelson Ko has made changes to allow users to get the desired results
BY USING THE PLUGIN LIST, thus avoiding what was potentially a REGRESSION that would have stopped anyone with such linked trackers from upgrading. The solution implemented by Nelson Ko is, according to Olaf-Michael Stefanov and Jean-Marc Libs, a fully acceptable solution.
While it involves some changes to tikis that use such item link and item lists trackers it provides a clear and clean upgrade path.

Noted and reported by omstefanov (olaf-michael) to TikiFest Montréal,
and documented in demo.tiki.org for TikiFest Ottawa, April 2013.