I got this as well.

Should be fixed according to changelog of 0.51 patch, but I didn't start a new 0.51 game yet, so I can't verify. Is your game started in 0.50 or 0.51?

Saw this with once with Verga in my 0.51 game.  He had 32 defenders and over half were guardian statues.

Here's a link to my saved game if needed.  https://www.dropbox.com/s/4rgig7nhfdw9lfl/Timbrat.EleSav

To reproduce take Lord Relias and attack Verga's city of Timbrat (sp?).  The city square will be full of guardian statues, and other defenders will be lined up off both sides of the screen.

Looks like its happening with multiple cities.  I need to look at my own because I haven't noticed.  I've been noticing it with enemy cities.  At first it was annoying, but now with assassin demons the tactical battle gets stuck at the beginning of combat.  Auto play doesn't work.  The only way to get out is auto-resolve, and that doesn't end well for me with only nine units against 32.