Over several decades, I have had several engine problems with symptoms similar to the OP with no codes thrown. In all cases, it took a very experienced mechanic with a "scope type" diagnostic scanner to diagnose the problem. In one case, I had a bad plug wire that was mis-firing intermittently. Another time it was a bad throttle speed sensor that sent bad signals to the tranny, causing symptoms like it was "slipping". No codes were ever thrown.
Ask your dealer if they have tried any testing (scope) beyond a regular code scanner.
Here's an example of a scope type diagnostic tool: skip ahead to see it in action.
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