if you use the speed spell from Ars Nouveau this happen

it look's like to me your going faster than the position update tick for the mod , this is normal for mostmod , when you go fast , it the same thing like the 1 tick piston glitch that causes it to leave the block behind the game doesn't update it fast enough by the time it receives it's false , there isn't much they could do ,except speed up the tick rate for the mod but that brings in more problems , i'm just wandering has this happen with vanilla speed potions if so they could possibly increase the frequency of the position update tick

note , what i mean by position update tick , this is basically updating a player position for the mod database, it doesn't exist as a command but a process you must do to keep it updated