Reviewing the footage, impact VFX was happening on the Moon Coblyn's snout (lower face), DoT VFX was happening on the core of the body.

About 21 seconds after application is the last tick of it we see, then it is absent for the three part combo. At the use of Demacia, you can see the impact VFX hit near the beginning of the attack animation (So server recognizes it), and then shortly after you see the DoT VFX once more on the core of the coblyn.

Seems to check out.