The service manual for the 617.95 engine states that an 80 degree thermostat should begin to actuate at 80 degrees and not fully actuate (main valve fully open and bypass valve fully closed) until the temp (at the thermostat) reaches 94 degrees. So it would seem that the thermostat that maintains the coolant temp at 80 degrees is the one not performing to specification.

Relying on the coolant temp gauge to evaluate thermostat performance is further complicated by the fact that the thermostat and temperature sensor are not co-located, at least not on the subject engine. When the thermostat is fully actuated, it is "seeing" coolant that has just exited the radiator, which is arguably cooler than the coolant to which the temp sensor is exposed.

In short, having an expectation that the indication on the temp gauge should equal the thermostat rating is not well grounded.