I think 1911 Tuner gave a pretty good explanation of what is most likely going on.
Let's look at what we know, and remember that during the entire process, things don't have to go completely wrong to cause the problem, just wrong enough, at the wrong time.
Last round fired case fed back into the chamber.
this means there are two things not happening. Fired case is not being ejected, and the slide is not locking open. And they are happening at what looks like the same time.
But, is it? exactly??
First, the extractor IS pulling the fired case out of the chamber. (and the rear of the barrel is pointed down when it does this -unlocked-)
And, this happens before the slide has moved back far enough to allow the slide stop to engage.
And here's where timing comes in. Where, in the process in YOUR gun is the slide at the point where it normally ejects? Is the slide in position to have the slide stop hold it open at THAT point??
I ask because I'm too lazy to go get my Govt model and play with it, right now
AND, because your gun might be slightly different from mine...
IF, as suggested the extractor is hanging on long enough to pull the fired case back and slightly down (and slightly depressing the follower) just enough, and at just the right time to keep the slide stop from engaging, and THEN letting go so the rearward movement of the slide does not kick the case out when it hits the ejector this could explain how your problem is happening.
Time to play with the gun... remove the recoil spring, so you can slowly move the parts by hand, and using a fired case, work the cycle slowly, watching to see where the slide is, in relation to the stop and when the case hits the ejector, and how much more the slide moves back after that.
Also consider this, might the magazine (follower) be playing a part?? Does the last round jam happen with every magazine?? Just some?
Do you have a GI spec magazine? Can you test with one? does it happen with one, or only with "other" magazines??
I know the GI spec guns pretty well. I know what their parts should do, and when. BUT, when dealing with a non-GI spec gun/parts the "rules" can become guidelines and tolerances can vary.
Just for example, look at magazines. Today you can have several different designs of followers, and a couple different styles of feed lips, 8rnds vs 7 etc. Point here is, things have been changed from GI spec. When that happens, the "standard" things may apply differently and you have to work with the gun in your hands to find out what, where, and how much the differences are, in order to make allowances for that.