Really, there is no other possible cause for the engine to be actually running too cold other than a faulty thermostat. You can easily test the thermostat by boiling it as I described. I have checked through many repair databases and for TSBs; every single hit in the repair databases (dozens!) agree that a thermostat is the only possible cause. All of the other techs in our shop agree as well. Many things can cause an engine to run hot, but the only thing that can cause it to run too cold is a thermostat.
So, knowing that there are only a couple of possible explainations for what is happening in this vehicle:
(1) You may somehow be measuring the engine temperature incorrectly, the ECM is getting a low temp reading as well due to high resistance in the CTS circuit, an air pocket in the head, or an internal ECM fault,, and the 2 seem to agree.
(2) You may have gotten a string of out of calibration thermostats (the odds are you did not, but the only way to know, is to do the test to verify that the thermostat is operating normally)
(3) Coolant is somehow bypassing the thermostat.
Logically, there are no other possible causes for what you are experiencing...