"Every 0.4s" doesn't work per each enemy sprites. It's like a system timer, which runs at strict intervals while there are any enemies in "attack" mode.
Since enemies enter the "attack" state at slightly different times (because you have random walking duration), for some enemies "every 0.4s" event will trigger three times, and for others only two times.
I suggest using Timer behavior for firing bullets too.
Also, the way you are moving the enemies is weird. You are restarting Tween on every tick in "Is timer running" event, this is not how Tween is used. I suggest replacing it with MoveTo behavior, and only start it once in the same events you start the "walking" timer.