Perhaps I'm wrong, but if this is the case I believe a stiffer magazine spring would solve this issue. Can anyone confirm this, or offer guidance from experiencing a similar issue and how it was resolved?
I strongly doubt a stronger magazine spring will do anything other than make you magazines more difficult to load. Might be of some use if your problem was your gun was not locking open when it should, but that's not what you described.
My theory is that the last round is shifting too far forward under recoil causing the follower to shift and engage the slide stop.
With the gun empty, open the action and allow the magazine to lock the slide back. 1911 pattern guns generally use the magazine follower contacting and lifting the slide stop into position to lock the gun open.
Check you gun and see if this is what it does. Note the position of the part of the follower that operates the slide stop, relative to the magazine body and feed lips when it locks the slide open. It should be at, or very nearly at the upper limit of follower travel.
Now, take the mag out, and put a round in it. Just one. Look and see where the part of the follower that works the slide stop is NOW. It should be well below where it was, due to the diameter of the round, and unable to reach the slide stop lug because the round in the magazine prevents the follower from moving high enough to make contact.
GO ahead and test your theory about the round "moving forward" allowing the follower to shift and contact the slide stop, by moving the round forward in the magazine (outside the gun) push it forward, do it in small steps, keep an eye on what the follower does.
My guess would be the follower goes nowhere, as long as the case is held between the magazine feed lips. Even if only the rim of the case is held by the feed lips, the follower won't go anywhere. Once the case is clear of the magazine, THEN the follower can rise up and work the slide stop, but not before.
What you describe, gun locked open last round IN the magazine, means that the magazine follower is not a likely suspect.
The most likely suspect is you. Only happens on next to last round, only when shooting heavy recoiling loads. One possible cause is that you are bumping the slide stop, (or. more accurately, it is bumping you) engaging it early. Possible to happen without you realizing it. This is simple enough to check, have some other people shoot the gun with that ammo and see if it happens when they shoot it.
The other likely suspect is recoil acting on the slide stop itself, overcoming the spring tension from the plunger holding the slide stop in position. Heavy loads, next to last round, meaning the gun is lighter than when fully loaded, might just be enough.
Take a close look at your slide stop where the plunger bears against it. Is it flat ? or is there a dimple for the plunger tip to fit into.
Both kinds of slide stop are found, which style does your pistol have??
Recoil can overcome the flat faced style more easily than the style where the plunger actually has a recess (dimple) to fit into.
Usually not an issue with standard round (9mm, .38Super, .45ACP) recoil levels, but it can be at higher recoil levels, which is what you are shooting.
IF it is recoil engaging the slide stop early, there is a balancing act you have to take into consideration. You need to find the right amount of tension on the slide stop so it stays in place under recoil, but not so much that the magazine follower can't operate it.
If your slide stop doesn't have a dimple, putting a small one in might solve your problem. If it does have a dimple, enlarging it SLIGHTLY (deepening it also) might solve your problem. Increasing the tension of the pluger spring might solve your problem, BUT BE AWARE that if you make too much of a change, the slide stop might get "locked" in place so the magazine no longer locks it open when empty.
Be prepared to purchase a replacement slide stop, if you go to far. Best would be to buy a replacement slide stop, and do the work on it.
OR have a gunsmith work on it, so that if HE goes too far, the cost of the replacment is on him, not you.
Good Luck, and let us know how it works out.