OK, here is my opinion.

Gas engines do not react to mixture like glow engines do. They either sag or fail to accept throttle right away, or keep running.
If a new gas engine sags after it first ran well from cold (3 minutes), this is caused by internal overheating. This way, new engines show they need more running time, more oil or both. Throttle should immediately be reduced, and the engine allowed to cool down. There is no need to stop the engine. If under these conditions you keep pushing the engine, something has got to go wrong.
I would also not consider this warranty, but a customer induced error.
Gas engines need to be run in properly. The seller should point that out well if queried.
I once got an engine back that showed the same symptoms. I offered to run it in for the customer. After repeated short power bursts and low power in between, bye and bye the sagging went away as the engine ran in, and it became a *****cat.