This bug is easy to reproduce.

1. Attack something with an army with multiple units (meaning your army has multiple units), or a champion that can summon units.

2. Let some of your units die in combat -or- summon some units during combat (Skeleton Horde works great for this).

3. After combat, but in the same season, cast Call of Titans.  It may have to target a champion, as both times I've been in this situation, that's what I was doing, but it might still do it if you target an empty tile.

4. The dead and summoned units are back and present in the created army.  They have 0 health, but they're there.  

If something attacks that army, those units won't show up, but this becomes a problem when you're trying to build a single army out of loose troops, and you instead get 2-3 armies because the main army filled up with dead units.  I lost units both times I was in this situation because I was using Call of Titans to stop an advance, but my army split into two wasn't strong enough to stand up to the advancing army and I lost.  I still won the game, but I lost those battles, which I otherwise would have won.