Its 10x worse if you play with the Marked Shot legendary ring, because you're used to it still being lit up every once in a while because of ring procs letting you shoot another Marked Shot, so you get all excited about shooting free marked shots and then it trolls you because its actually just bugged. This happens most often on the slimes on Helya, and the explody mobs on Xavius. Basically if an add disappears from some other way other than actually dying while you have a mark on it, then it will bug out your shot until the timer would have fell off naturally.