Reddit user u/darkjedi39 noticed and made this post recently, showing how the lip of the Rival mags has changed (something I did not notice myself on the Hypnos). Indeed, it seems that all newer Rival magazines are being made like this. The reason has nothing to do with the Nerf blasters themselves, as they don’t grip on that surface in any way. Instead, it appears to be a small change to prevent current Dart Zone/Adventure Force blasters from being fully cross-compatible.
The Accelerators, Titaniums, and other ball blasters made by Prime Time Toys were made to be able to use Rival magazines, by gripping on that lip. With the molding change, however, that is no longer possible, at least not in stock form.
Assuming there is no change on Dart Zone’s end, users will need to remove the extra plastic bits as shown in the picture to restore cross-compatibility. That is, if you haven’t already decided that hoppers are a better choice for you.