Surecast has this effect, but Arm's Length doesn't because melee jobs did not used to have casts except PLD. PLD got a trait which prevents cast interruption upon getting Holy Spirit and now in most cases they are instant casts.

I do remember not being able to do much about it on a Samurai in the past, other than to delay using it and start another combo while the knockback is being cast. Maybe a similar trait would be useful for them.