Welcome to the forums.

A few thoughts....

Is the radiator core clean and free of any dirt or debris that could inhibit airflow.

Any bubbles in the coolant reservoir?

Is there any brown-ish milky goo under the oil cap, or does the oil look milky?

Are there any signs of white-ish exhaust, or the smell of engine coolant?

If so, you could take the plugs out and check them for signs of coolant, or inspect the cylinders with a camera.

https://www.jeeppatriot.com/threads/fuel-injectors-leaking.342605/page-4#post-1896932

There's also dyes around that can be used to detect the presence of combustion byproduct in the coolant.

 

Your secondary thermostat could be stuck closed. It's supposed to open at 203 deg F. to provide additional cooling. The primary stat opens at 173 and feed the upper radiator hose, so if a mechanic wasn't aware our engines have two thermostats, they may just feel the upper hose is hot and assume the thermostat (singular) is good.

 

Also check your cooling fans. I had one stop working and the engine would overheat at slow speeds or idle

 

no leaks and no evidence of a blown gasket would indicate (to me) a bad radiator cap. it has a 2 way valve to let fluid in and out of the coolant reservoir (overflow tank)(whatever). if it lets coolant in to the overflow tank, but not out, it will eventually fill the tank and spew overboard. now the coolant in the engine is low. guessing the mechanic doing the testing "fixed" it, maybe without even knowing it