Nope! Not bugged, just high-variance drops on a janky and intensely punishing fight. The common wisdom is to bring an Iron Slug or Winter Grave to deal with them quickly, as Magnus charges will plow through their large hitboxes to easily hit 3-4 times (Iron Slug because these Tortodrones are not actual constructs, and resist all non-normal damage; Winter Grave because the freeze helps a lot).

Sometimes you'll get two shells; usually it's about three or four. I think I've seen as high as seven or eight, though very rarely. It's very unfortunate that this is the state Tortodrones exist in, and even more so that the rewards you get for throwing yourself into the meat grinder of this event are not particularly useful. And the 10k cr entry fee is a lot, even if you split it with your party (assuming your party even has the courtesy to split it) 2.5k cr per go adds up.