Hm, I would like to see them 1v1. It's not much of a surprise to me that shields lose in this type of situation.
Here is my speculation based off experience- The AI's ability to block without a shield is pretty good, especially for a high tier unit like sergeants (if this mod uses latest AI patch) and all you can really do in these clusters is overhead swings anyway. And perhaps without the shield they are more aggressive, which tends to work better than defensive in the cluster. My reason being that shieldwall formations seem to always lose against opponents in more offensive formations like line.
Shields are much more useful in 1v1, and of course for blocking projectiles.