Quinn using passive at max range on a marked target SOMETIMES does not trigger a harrier proc
Very rarely you can have a situation where at max or near max range (possibly after using another spell or ability or something) Quinn will shoot a normal arrow at a target with a mark.
There was a bug similar to this one before, this one is identical except rarer and I don't know the cause, it's been around forever though.
It always seems to trigger after Vault though, I'm not sure why.