Need verification of this, haven't found the chance to duplicate and suspect it was a save from a previous version.

I lost the save prior to the attack but I chased a troll army down and auto-resolved the battle and each time it would only show the last unit in the line killed (my army was extremely offensive focused). 

Chase them down and auto-resolve again, the last unit that died last battle indeed was gone, and now the new bottom unit was killed.  Was a group of 4 trolls to start with and took 4 battles to finish them off.

I believe I got experience for killing the entire stack each time, IE first for killing 4, then 3, 2, last one.  And unfortunately I was so confused what was going on I forgot to check the combat log, but I suspect it was accurate.