I have no idea, especially as I usually use "next soldier" quite a bit.
Maybe because I was too OP and didn't bother much with tactics. I had a Devastator full of mostly gals in Blitz and Annihilator armor, so I just had them get out, 3 or 4 defended the Exterminator with reaction shots, the others shot what I could see from where they were. Most gals had deathblossoms with HVAP, the others X-Gauss and one Autocannon.
I only had to fly around a bit to find the last 1 or 2 zombies, and I used just 2 gals to search for them. No need to bother with more if they can't touch me anyway.