UNIT - Creature Balance + Visual Changes

Is this for sure? It smells like a bug, both from a technical point of view and from the point of view of elementary logic.

Absolutely, but you can benefit from this bug if you name the ability correctly (enhanced attack or defense penetration) and give it to melee units as well, setting the knockback trigger factor to 0 (disabling knockback). In general, regarding the upgrade equations, the common concept is to balance them based on the strongest upgrade, but in my opinion, it should be balanced based on the first upgrade, as it appeared earlier and is the base. For example, if we exclude the forest archers, it is unlikely that anyone would think of increasing the damage of the first upgrade, as it is already high (elves are one of the top-tier units in their tier and do not need to be buffed). Or, if we exclude the alternate human archers, few would want to significantly buff the Arab archers - they are quite decent archers in themselves, perhaps only slightly adjusting the ability to a range of 4. And so on. After all, before the alternate versions appeared, there was an established balance for the first upgrades, and it makes sense to focus on it, considering the overpowered direct-fire human and elven archers, which need to be nerfed (in terms of stats or abilities), and other alternate versions that stand out from the balance, as bugs. I think that this point of view (balancing based on the strongest upgrade) is adopted because it is more pleasant to buff than to nerf, but this is clearly not a professional approach to balance.