Let me start of by saying that I really enjoy shooting the Glock 21 SF. I am hoping to get its cousin, the 20 SF in 10 mm soon. Same frame and slide.
I am not a fan, however, of that ambidextrous mg release. It looks really good on paper, but you loose (well I loose) the ability to do a rip/rack to clear a double feed with a closed slide (at least as closed as a slide can get with two rounds trying to get into the chamber at the same time). Whereas with my Glock 26 or Sig P220C, I can clear double feeds by pressing the mag release and just ripping out the mag, the Glock 21 SF requires that the slide be locked back first.
So that little gripe out of the way, I am wondering if perhaps, the mag spring is not somehow sitting in a jacked up way inside your mag body. Perhaps the follower is not entirely seated in the spring, or maybe the spring isn’t fully seated in the base plate. I haven’t had any 21 SF mags apart, so I don’t know the precise inner workings.
By all means call Glock and let us know what they say, but you might disassemble one of the mags and see if all is well in there. This task is virtually impossible unless done just right. Search youtube or something for a video, to get an idea of the steps (all you really need is a punch).
ETA: Using a brand new ambi Glock 21 SF and any of six brand new magazines it never took any undue force to load a mag into place. The more I think about this, it sounds like the previous owner may have added some extra power springs or some other such foolishness. If I were in your shoes I would order a dozen extra mags (or a half dozen just to have) and if they work fine then send the goofy mags back to Glock for replacement or repair if they will do it. I'm guessing they will.
When doing research for my quest for 10mm, I spoke with a CS rep at Glock. I think their warranty is somthing like 80,000 rounds or 1 year whichever takes longest to reach. I asked him to repeat himself, and he did indeed say longest. He might have said 50k rounds, I don't know, but I do remember thinking, "Damn! that's alot!" I've never had to use their warranty, but they do seem easy to work with.
ETA2: After reading the post following mine it got me thinking. I might be wrong, but I believe that the 21 SF with ambi mag release only works properly with the newer mags. I don't know if that means that the ambi release only works with the new mags, or if they don't work right at all.
The ambi release mags will have a slot (the ones I have seen also have some metal showing through) straight across the front of the mag body, whereas the old style only have a notch there on the right hand side of the mag.