If I may, I think your definition of "bug" is incorrect. A bug is simply an unwanted behavior in a program. I have great difficulties believing paradox intentionally wished for the decision to work as it does now.
I will grant you that the cancelling of the decision itself, is not a bug, but rather a catastrophically poor UI choice combined with the illogical - but possibly intended, although I cannot imagine why - fact that the start conditions and reverse of the instantly cancelled conditions don't match allowing them to be both fulfiled at the same time which shouldn't be possible.
However, the fact that upon cancellation, the skirmishes modifiers aren't removed, in spite of the fact that they represent your divisions - that you could have all removed - fighting there, and additionally, the fact that the decision cannot be attempted again if it cancels - I am quite confident these can be considered a bug, for it makes no sense for paradox devs to have wanted such a behavior.
In short, the decision itself isn't bugged, nor is its cancellation itself ; but the consequences of its cancellation are, indeed, a bug. The fact that the decision can be completed without being cancelled and thus without encountering the bug doesn't remove the fact that a player may wish, for whatever reason, to move a division away at any time, cancelling the decision, and thus running into the bug.