This is linked to the old bug of being able to move and attack with one movement point (or two if you were two spaces away).

Now, when you try to attack a unit, if you have to move a space to do it, and the unit is out of movement (not actions), their turn will end. Next turn, they will be given only the number of Action Points they had remaining when their turn was ended.

In the case of typical Tarth warriors, they have 4 AP, 2 of which can be allocated to movement. If you select to move two squares next to an enemy, then select to attack them, it functions normally. If you select to attack an enemy which will require you to move two spaces, you will move and your turn will end. The following turn you still be given two AP, instead of your normal four.

This has been consistent and repeatable throughout several combats, with Trained Units, my Sovereign, and a summoned Sand Golem.