The reason why some lose the shield and others not has nothing to do about lag or delay but the fact that when you AoE heal the heal you do won't be equal to everyone. You will notice that some will get normal heal and some get crit and even those values will differ between the players, like the hit is around 3000 and your normal heal is 2500 and then you crit for 4000, so who got crit will keep shield and other will lose it.