European servers

[Alukah]

This is because the effects of an ability are tied to the animation, so the DoT will only become active (And therefore subject to Bane/Fester) once its animation completes. This is a design choice by the devs and not an issue to do with lag or latency at all.
Such a lack of understanding shows you might not know what you're talking about in other areas.

"I am right and you're wrong" is all you're saying.

Bane/Fester right as the debuff icon is applied and the game may still fail to register it.

Also buy a dictionary and find the definition of instant.