Definitely a case of air in the system, if the stat checks out okay, but easy to bleed following the correct procedure.
I can send your garage a link if that would help them.
Looks like you have the temp sender unit installed into the top hose and I assume an aftermarket gauge is fitted in the cab?
Might be a plan to hook it back up to the OEM sender unit at the rear of the engine to see what change occurs as the top hose temps are different from the OEM position at the rear of the engine.