I have heard speculation that it has something to do with the crank vaporizing the oil at that level when run hard. Which would explain the particulars of the problem. I don't know myself why it happened.

I can tell you that I have seen it many times. On stock engines in particular. We used to have to keep a sharp eye on our oil level. It needed to be about a quart low in order not to be adding oil regularly. At the same time, we didn't want it to get much lower due to the risk of losing oil pressure under hard accel or decel. Fortunately there usually wasn't much change when kept a quart low unless the engine was getting tired.

But the top quart of oil in big blocks had a nasty habit of exiting the engine some way or other. I have never experienced that with any small block. Has anyone experienced that with the small blocks?