Self-healing Hog eats my sleep dart

During this game (92YYDE) I get forced into an awkward spot on point A of Anubis whilst playing as Ana. At 2:10 roughly i try to sleep dart the enemy Roadhog but the dart seems to vanish when he uses Take A Breather. Am i going mad or is it a magical disappearing dart? Unless i’m mistaken and Hog’s ability is meant to do this but i’m pretty sure it isn’t.