Not the health bar. You get lightning effect on one unit, another one does 'ouch' sound and loses health. It looks like game randomly picks one unit to show lightning effect and then again randomly picks unit to apply damage (effect). Can be same unit if you get lucky with rolls, that's why it's better to have more units on your side to spot this bug.
Btw, it's a bug. Randomly hitting enemy unit does not mean that you see graphic effect on one unit and see another lose health.
Unfortunately, I ran out of air shrills in my current game but any save with 4+ army attacking several air shrills would do. Just attack and watch their first move.