EnterimCollaide[...]answering this a day late: that's always been in the game because the game's definition of "self-inflicted blast damage" is more specifically "self-inflicted blast damage that doesn't damage any other players". blast jumps that splash damage enemy players even a little bit don't get the self-inflicted damage reduction for that reason.
Along with that bug, I would also like to know why soldiers with gunboats deal 80 damage to themselves if they also hit an enemy (even if they perform a rocket-jump, which are said to have reduced self-inflicted damage according to the item's description)
I know how it works, but why though