Actually, there's two mag problems with the S20, the first is the one mentioned above where you get a jammed feed, IE the bolt doesn't travel quite far enough rearward, grabs the top of the round by friction, and it jams nose up. The fix above does address it, it's just a spacer that moves the rounds slightly forward in the mag, you almost certainly lose some max cartridge length. From what I've read this is actually the less common issue.
The more common issue is the rear of the case never comes up high enough in the mag for the bolt to pick it up, the bolt travels far enough rearward, but the rear of the case stays down in the mag. I had this problem in my 300wm. It seems more popular in the longer action calibers. Sako evidently addressed this at some point with updated mags, but I've never see anyone outline what the change is. The higher capacity mags are supposed to have less issues, no idea why, stronger spring would be my best guess. In my old mags you can just press down on the center of the cartridge and the rear will just stay down. There's WAY more spring pressure on the front of the cartridge than the rear.
The downside is now you have to deal with Beretta, and they are a serious wastes of skin and oxygen. I contacted them about this issue a few months ago. They told me that they had replacement updated mags in stock, but had "no policy to replace them", even though they admitted there was an issue and they were aware of it. They said at some point update kits might be available, but even though they had updated mags in stock, their solution was to offer me 10% off so I could buy them at MSRP in their online store.