Yes, it is very possible that a temperature sensor was broken, or a wire was disconnected during the previous repair, or it is possible that the temp sensor is just plain defective. However, the cause of the problem is not your concern as it is a warranty issue, plain and simple. EVERYTHING on your car is covered by the Bumper-to-Bumper Warranty (Subaru calls it the “Basic” warranty) for 3 yrs/36k miles, and the engine and transmission are covered for 5 yrs/60k miles by the Powertrain Warranty.
Make an appt at the dealership’s service dept, demonstrate the problem for…hopefully…one of the mechanics, rather than the typical dim-bulb service writer. If necessary, ask to speak with the service manager. If these tactics are to no avail, contact Subaru Customer Service at the corporate level. Contact info can be found in your Owner’s Manual.