So, It appears that when there is one shot left in the magazine, mulligan will often pull a magazine sized portion of rounds from reserves, but then load only a single shot. This is an especial case in crucible, and I have a few recordings of it happening on Beloved specifically if proof needed (had it occasionally on other weapons in the past but just passed it off as some random "glitch"). #PvP #Balance #Feedback #Destiny2
Sounds like you've got drop mag on your sniper especially if it happens in Cruci where you pick up special like candy and reload right after it might seem like your one or maybe two shots you have in the mag vanish and turn into only a single shot (2,3 shots if you've got scavenger perks) may seem like a bug, but it's actually the player having missed their weapon having rolled with drop mag though it's still a mystery to this day why you'd want this on a special weapon.
Maybe you have drop mag?