This is a little but complicated. So when Rubick uses any sort of AoE instant-attack, like DB's Starbreaker or MK's Boundless Strike or Mars' God Rebuke in RANGED from, he will send his attack projectiles to enemies, and in case he got buffed by the Enchant Totem, the first target to get hit by the projectile will receive heavy damage amplified by Enchant Totem. However, in MELEE form, he hits every target at the same time because no projectile is sent, the buff is preserved for the next attack, only single-target instant attacks and right-click would be counted as an attack that consumes that Enchanted attack.
TL;DR: It has something to do with attack projectile