This bug is weird. Really weird.
If the heart has bugged out, you’ll approach the area and possibly see a few suspicious trees/bushes, and some warhounds attacking the air. However you’ll quickly run out suspicious trees to kick.
If you start swinging a melee weapon near the air-striking warhounds, or use a “trap” ability that triggers when it hits an enemy, you’ll notice that melee will chain and traps will trigger. Do this enough times and you’ll end up killing an invisible enemy and get credit for one. Immediately after, all other related enemies (ie: kill one separatist, his two partners show up, maybe with less HP) will suddenly become visible.
Of note is that it takes a disproportionately large amount of time to kill the invisible enemies than when they’re visible.