Grineer Submarine + Cleaving Whirlwind + Zenistar = broken melee combos?

[Hjalmthrimul]

I recently acquired a Zenistar, and it's instantly hit my favorites list.  However, it has some decidedly odd interactions with other mechanics.  The oddest of which is, after falling into an archwing pond in the Uranus Submarine tileset, the movement associated with some combo attacks is reversed.  As near as I can tell, it only happens after you've thrown the fire disc using a charge attack, but it doesn't matter whether you leave the disc out or retrieve it.

Tileset: Grineer Submarine
Frame: Hydroid
Weapon: Zenistar (Cleaving Whirlwind)

I have included video to explain better than words how the motion changes.  For all the attacks, I am pressing forward while attacking; the repeated vertical strike combo doesn't work otherwise.