The mechanics have changed. Splitting stacks no longer increases defensive damage or damage reduction. For defense there is no difference between a single stack and multiple stacks within 5km of each other. One exception is that, while the total HP taken and given in a particular round doesn't change, splitting stacks will affect when units are lost. Thus, the results of the battle will be slightly different. As an example, imagine you have 4 infantry defending at 50% health (hp:40/80), then it's defensive damage potential would be:

4 * 5 *( 0.05 + 0.95 * 40 / 80 ) = 10.5

Since each unit here has 10 HP let's say the attacker does exactly 10 HP of damage.

If all 4 infantry are in the same stack you lose 1 so the damage potential for the next round would be:

3 * 5 *( 0.05 + 0.95 * 30 / 60 ) = 7.875

If instead you had 2 stacks with 2 infantry in each you wouldn't lose any infantry in that round so the defensive damage potential in the next round would be:

4 * 5 *( 0.05 + 0.95 * 30 / 80 ) = 8.125

As you can see, in this round of this particular example, the two stacks of two infantry would do a bit more defensive damage.

It's conceivable that a single stack could have an advantage if it has, say, between around 30-50 units. In such a case, losing units faster could have a positive effect of raising the average HP so that the 20 full damage units do more damage. One would have to do some calculations to compare with multiple stacks or run some simulations in the battle calculator.