2013 Ford Fusion Overheating Only On Hills, Highway, or sitting in stopped traffic?

I have a 2013 Ford Fusion SE. 5 months ago it overheated. I took it to shop and they replaced hose. Was fine after that for few months. The past month it has been overheating/running hot according to temp gauge. Also had check engine light on which when checked, had codes for the Coolant Bypass Solenoid and an Emission Purge valve. The temp gauge only rises above normal temp when going up hill, on highway, or sitting in stopped traffic. Often the temp gauge goes clear up to hot and alarm sounds. Soon as I crest hill, or drop my speed on the highway/take my foot off gas, or start moving again the temp gauge goes back to normal. Took it to shop last week and he hooked it up to machine that measures temp and confirmed it was hot. He looked it over and said the hose that had been replaced previously had come loose and was leaking coolant, so he tightened that back up. He added 1.5 gallons of coolant because I was out of coolant. He hooked the car back up to his machine that checks temp and drove it and it said temp was normal, nothing else wrong. Picked up the car and took it on the highway and up the hill and it overheated and alerted us that it was hot. So we pulled over, boyfriend opened the hood and touched the engine and it was just warm, definitely not hot like it is if the car is overheating. So we're taking it back to the shop this week. Fans work, we hear them kick on. What could the issue be?

You were out of coolant? Don't you check fluid levels? If not you will end up with a scrap engine which will cost $3000+ to replace. A good shop would test the coolant for products of combustion which is evidence of a head gasket leak. To need 1.5 gallons of coolant means it's going somewhere. Probably into the combustion chambers and out through the exhaust.

It HAS a code for the coolant bypass solenoid. Check what is wrong with it. It is the likely cause of the overheating. You can also ask if Ford if your car is covered by a recall for this failing solenoid so a dealer can replace the solenoid for free. And you might also want to see another mechanic because your last mechanic does not seem to have a clue.