Experienced this many times now while playing co-op, the animation simply doesn't play when using the drum magazine. Switching to sidearm and back fixes this temporarily but can happen again later. Don't know if it matters but I was using a foregrip, compensator and cobra sight and also 1.5x sight.

Also little personal feedback on the animation itself (drum mags in general), to me it looks unnatural when dropping these mags, like they fall too slow/twist like it's low gravity