Once an error is triggered under the regulations it must be stored in memory for some timeframe, even when the error has been resolved.

To clear that error, X amount of TRIPS must occur before that error will be erased by the PCM.

OBD-II Warmup cycle

To complete an OBD-II warmup cycle, the engine must reach 160 degrees and achieve a temperature rise of at least 40 degrees from the initial startup engine temperature.

For exampleL a engine could start at 120 and rise to 210 degrees and achieve over a 40 degree change, in this case, the warmup cycle conditions would be met.

However, if a engine started at 150 and rose to 180 degrees, a 40 degree change in engine temps would not have occurred.
In this case, the warmup criteria would not be present and some OBD-II diagnostic tests would not be run.

Thus if you were to install a lower degree thermostat that prevented the 160/200 degree rule, it would take much longer a time period for PCM to relearn and to clear errors stored for XX amount of trips which can be as high a 40 trips.