Not an anomaly in the context of frontier warfare no. Montcalm isn’t implicated as orchestrating it or allowing it however. For instance the garrison of Fort Loudoun was attacked in much the same way after surrendering to the Cherokees in 1760, without any prompting from a European ally.
At Oswego, which was an unauthorised and unexpected outbreak of looting by some of the allied tribes in which some prisoners were murdered, Montcalm acted swiftly to end it and put the British under guard even though it would cost him money and materials to do so.
At William Henry he again attempted to stop the massacre and had taken steps to try and make the native leaders restrain their men. Unfortunately the Indians were there from several tribes all fighting their own war for their own reasons and didn’t fight by the same rulebook.

Therefore the incidents are more evidence to show that Montcalm couldn’t control the various tribal allies rather than deliberately allowing it to happen because he didn’t have the men and manpower. He was careless at Oswego, but wrote that despite the cost he would be obliged to protect the enemy prisoners from his allies. The massacre isn’t the anomaly or the error, but the scene were Montcalm arranges it and gives Magua free rein is in purist terms inaccurate.

Josh.