This was called WAI the last time a dev commented on it I believe for balance issues, they're both typed as a weapon buff similar to Kensei's One-Cut. Cetus used to raise the issue that this also meant that they could not stack with arti buffs like deadly but they changed the typing to allow it.
If you look at the combat log it definitely looks intentional with the Remove Temporary Enchantment portion.
You raise a good point about the 15 second duration though, tooltip says it's supposed to be 10 but also lists a 30 second duration for some reason.
VKF T5 has been pretty heavily buffed recently and it was already super solid for a universal tree. Blessed Blades (T5 triple hit cleave) and Rapid Slash (T1 triple hit cleave) animation cancel the other so you can chain two instant triple cleaves that deal +30% for 3 aoe swings and +50% for the other 3 aoe swings (Plus they don't even check the offhand) while exalted cleave (T2 1 hit cleave) was nerfed down to 10% and avenging cleave (T5 1 hit cleave) was nerfed down to 40%.
This is now the bargaining stage of calling for cleave balance.
(totally not a bump or anything)
