It sounds like the Bane issue is not an animation lock or server latency issue but a DoT state check issue.

From what I can gather on the way DoTs work in this game, the initial use of the ability tells the DoT to be applied to the target but the actual state of the DoT being active upon the target is tied to the DoT timer/ticks and so is not registered as the target having the DoT until the first tick at which point the DoT is "active", starts it's timer, does it's damage, etc. Therefore if you use an ability such as Bane that checks to see if a certain DoT is active before the tick has happened and the DoT is actually set as active, the check fails and the ability doesn't take that DoT into account even though the player has applied it.

As far as fixing the issue, well in a ways that is just the way DoTs work in this game and so potential different solutions would have to be attempted.
Off the top of my head the two solutions that I can think of are either add a delay into Bane so that a tick is guaranteed to have happened between the time the player activates the ability and it goes off, although this would likely make SMNers very unhappy, or possibly add a secondary hidden "debuff" aspect to DoT's that doesn't have any affect (or effects) that gets applied immediately, is independent of the DoT timer/ticks and acts solely as something for the "active" check to work off of instead. I'm not fully certain if the latter would actually work though since I'm not too familiar with their engine, dev tools and the limitations of them.