Can you tell if its primarily mammets or other playing characters?
I noticed even when I stand directly in mammets, they sometimes are out of range for me.
If gapcloser abilities like onslaught travel distances greater than the maximum range there is a possibility you will dc, its the same with MNK.
It's funny because if a target becomes untargetable in the process of the skilluse, it will not go on cooldown but your character already traveled 1/3 or 1/2 of the way and you get the opportunity to use it again. Which results in even bigger distances traveled.