It is most definitely not be design, nor does it have anything at all to do with spell interruption chance. If you start to cast a spell and jump immediately, 100% of the time, no exceptions, it will interrupt the spell. If you start to cast a spell and jump at the very end, right before it finishes casting, 100% of the time the spell will complete anyway. This is pure server lag.

I haven't tried the Goblin server but I suspect the same issue is there as well, just some people don't notice it, particularly if they play a class without casting times or haven't played a more responsive MMO to compare it to.