@SorryHaah is wrong about this right-click bug being in game since release or beta. It was added to the game in latest patches.
Since first version of the game in 1999 right-click has made units try to find compromise between attacking targeted unit and splitting between nearby units. This behavior might seem a little bit unintuitive, but it is needed in order to keep right-click working properly when ordering units to attack group of enemy units. Units must be able to attack closer units in order for group attack to work without units overprioritizing reaching single targeted enemy unit.
Everyone can test right-click behavior in AOC/HD scenario editor for themselves. Go on, arrange 40 vs 40 melee units battle, select your units, right-click on enemy unit and observe what happens. Similar behavior must happen in DE so that people can continue playing like they are used to.
I even answered to @SorryHaah in that topic, asking him to compare AOC and DE behavior, but he didn’t respond.
Recently a bug was introduced, which causes right-clicked units to not prioritize attacking targeted unit enough, but hopefully their behavior will be returned to AOC/HD behavior in next patch. You can see from that topic, that after the bug introduction suddenly a lot of people started to complain about it.
Before the bug only couple of people complained, but during that time right-click worked similarly to how it works in AOC/HD, so they simply don’t like how right-click has always worked in AOE 2. Tough luck, it is fundamental part of gameplay and should never be changed.