Something similar happened here a few years ago on a GM vehicle.
Car came in with broken timing belt; checked everything as per normal, looking for a cause other then just an old belt; all seemed good. Car left running ok; came back a couple of days later on the back of a tow truck. This time the belt wasn’t broken; but a number of teeth were missing (stripped) on the new belt; allowing the engine to go out of time.
Replaced it again; but this time we left the timing cover off, so I could see the actual belt turning and easily check belt tension after the engine reached operating temperature.
After shutting off the engine; I attempted to rotate the engine manually to check the timing. Found that it took twice as much effort to turn the engine, then what it did while installing the belt with the engine cold.
After removing the timing belt (engine still hot from running) we found the camshaft was the cause of the resistance. Once the engine cooled it turned normally; ended up having to replace cam shaft and bearings.
The only thing that made sense was that the cam; when it became hot would warp and cause it to bind.
Not saying that this is what’s happening with your vehicle; but maybe suggest that they check the amount of effort required to manually rotate the engine while it’s still hot?
Dan.
Canadian "EH"