At first I though maybe it is desync (like it happens with Iron Slug charge attack when bullet flies but does no damage) but this seems to be consistent and based on distance to the target and attack angle. Target has to be right in front of the player and a bit to the side for this to happen.
Easiest way to reproduce would be with Shufflebots in training hall. Especially common during a fight with Mecha Knights when they start charging at player and continuously walk in place towards the player. Mostly happens with other alike monsters which tend to move towards the player during their attack: Lichens, Wolvers, Greavers, etc. But can also happen with any other monster as long as player and target are right in front of each other (i.e. it is not possible to get any closer). This doesn't seem to happen with Blitz Needle.
Thanks!
Today I was able to reproduce it with Grim Repeater and found even easier way: in training hall get cornered by Shufflebots in the bottom left corner, charge the gun and start running in place into bot on the right, release the charge while running. I guess it has something to do with the knockback, it is the main difference between Pepper/Reaper and Blitz behavior.
Hope that helps!