I did the 'Esuna test' a bit more, but I used mount chocobo because it has a very distinct result when the spell succeeded. Although this game is laggy, it is not common to have an entire second of lag, so in the interest of saving time here's what I did:

1. Cast mount chocobo.
2. Auto-run on with 0.5s left (this basically simulates a 0.5s casting).

Now there are obviously some room for error based on my reflexes, but what usually happens is that I start running at 0.5s, the spell castbar finishes, but I do not get on the mount.

This means despite the fact that the 'spell interrupted' message never came, the server did get the fact that my position has changed and thus did not allow the spell to complete. The client was not told of this, but the outcome is consistent with what you'd expect (shouldn't be able to cast a spell while running). The server doesn't really care that it interrupted my spell without ever telling my client that it was interrupted, which is consistent with the assumption that the client has basically no power to do anything to begin with. In this case, it isn't even being told the spell was interrupted.