Is this actually a bug? I've just assumed it was part of gameplay and considering it a must-have perk for casters.

One thing that I think IS a bug is the combo of the impulsive ability and the 'haste' spell. If an impulsive caster casts 'haste' on themselves on turn 1, they will get one extra move. If they move their first turn, then take their second and THEN cast 'haste' on themselves, they will get a 3rd move.

I think the initiatve list is somewhat bugged in general at the moment.

Example, casting slow on an enemy (successfully) will often rearrange the order in which my units take their turns with respect to one another.  (Hasting a unit often has the same effect but it is not so easy to discern since the order of my units can be expected to change - but not the relative order of those units which are not hasted, and yet they do, at least at times.)