I have seen tutorials, with pix, on scoutrifle.org from posters like yourself who are having trouble with some of their polymer mags.  Obviously, one alternative is to contact Ruger, as you did.  At least they sent you mags worth a good deal more than the one in question, but I understand your frustration.  The 3 -rd mag might be required for legal hunting in some, if not many jurisdictions, so it is by no means worthless.

In the tutorials, the poster cleaned up around the latching area on the mag with some alcohol, and after it dried, applied just a sliver of JB weld to the end of the tab where the finger-actuated locking lever on the body of the rifle engages the tab on the magazine.  Careful filing and shaping of this sliver of JB weld serves to raise the rear of the magazine, thus eliminating the problem you have.   The mag body needs to be raised until the top sides of the mag are touching the mating sides of the rifle magazine.  If you install your problematic mag, and look inside the body of the rifle (bolt open) I'll bet the mag body has some little bit to travel upwards before it touches the corresponding part of the rifle.  Your mags that work well probably latch-in considerably higher than your troublesome mags.  Although a PITA, this solution at least is mag-specific, and seems to be long-lasting.

Apparently this is some sort of problem with tolerance stack-up between various components which interface at a certain point, this being the the mag latch.  Shouldn't occur, but sometimes these things happen.

I am sorry to hear about your QC problems with Ruger. I hope they have been addressed to your satisfaction.

I have been doing some reading on scoutrifle.org about Ruger steel and poly mags.  Most people have had no problems with either, but there are some with occasional issues with one or the other types, and usually with the odd magazine, like your problem.

I looked for the tutorial on Scoutrifle.org, but was unable to find a link.  Some diligent digging on your part might well find it, and I suggest that approach highly.  Much info pertinent to your mag problem there.

I didn't take it that you were bashing Ruger at all.  If a person has a legit problem, that is due to some manufacturing shortcoming, posting about is not out of line.  What is not often seen is the progression from detection of the problem through return to the mfr, and a report after the return of the firearm.  You did that with the mag issue, and your frustration aside, I think you will agree that Ruger's response to your problem had some comical aspects to it.

Do look around on scoutrifle.org, and you will find the tutorial about the poly mags.  At this point, I think that the best way for you to go.  Again, while a PITA, you will wind up with a GTG mag for a little time on your part, and peanuts for cost.  You ought not have to do this, but there it is.  Good luck, and when you find the tutorial on the poly mags, please post it here.