You pretty much answered your own question, I think. Just like sleep, it all has do do with whether the debuff hit before or after their turn. Round doesn't matter. So if you immobile them before their turn, they will be up and ready the next. If you do it after their turn, they will miss their next turn/round. The way to look at it is, immobile will make them miss one standard move/attack unless removed by a healer/debuff remover.