This thing is over a year old and is as random is it can be. Most noticable when shooting at a Junker on Fjords, so it takes extra long to get the kill and it is extra easy to see the bug.
Best guess I got is that the animation is wrong or some internet connection decided to troll us all hard. But then again, any other ammo type on the Lumberjack seems to deal damage as expected, especially noticable in hardcore long range battles and no other weapon has been found to have this issue.