This is my one complaint about Subaru. I've had  2010 and 2013 Outbacks and my daughter has an '09 Forester. The thermometers for outside are never correct!  Today it's 93 out and for the 2 hours I was out and about the gauge read 73!  (The temp in my garage, maybe?) This happens just about all the time!

I've never seen the external thermometer on any car be very accurate.

They were on my other cars--Volvo, Audi , Honda Pilot, I think the Ford Explorer. True-it took 10 minutes , but this is different.

I haven't seen any real issue with the outside temp reading with the Soobs we have had. They all read close to the actual outside temperature. You should be able to get the trouble fixed. Hopefully there is just a bad connection to the sensor causing the trouble. There may be a proceedure to check the calibration of the circuit by replacing the sensor with a resistor value of some sort but I don't know for sure.