I'm wondering whether there is any chance of the following. The empty case, still held by the extractor comes rearward with the slide until the rim of the case strikes the mouth of the next cartridge in the mag. When it catches enough, this bounces the rear of the empty case upward and out from under the extractor. The empty case thus never hits the ejector, the slide fully cycles and picks the next cartridge from the mag -- JAM.
This scenario might even explain why the problem shows up most frequently when the pressure on the cartridges in the mag is at its highest. Would have been interesting to see the failures with 6+1 (cycling with a full mag).
To test, you could put a bit of fingernail polish on the top surface of the top round in a full mag, chamber a round from a different mag, swap mags to put in the "painted" one, vigorously hand cycle the slide to the rear without allowing it to come forward, and lock the slide back. Now inspect the polish on the next cartridge to see if the case mouth was hit.
Worth a try?