Against my better judgement, I will respond one more time.
The empty fails eject when the extractor fails to hold the extracted case against bolt face. If the case is not held against the bolt face, the ejector spring isn't compressed. If the ejector spring is not compressed, the ejector cannot eject the case. Call it what you want, but the root cause is still a failure of the extractor. Therefore, I call it a failure to extract. A loose case in the action blocks the feeding of the next round from the magazine. When that happens, the bolt pushes the round partially from the magazine. However, the loose case stops the round from feeding completely and the bolt pushed forward and up and over the rim (base) of the fresh case. Bolt over base. Sometimes, after causing the bolt over base, the empty is pushed out of the ejection port. This leaves only the bolt over base. It's no more complicated than that.
My research is based on hands on application of decades of professional troubleshooting experience and techniques. I don't care what the internet says. I do not trust what the internet says. I don't care if you think I'm full of it. I care about getting machines working right in the real world. I know what my successes and more importantly, what my failures were and how I worked through them.