Hey,
This is not actually a bug, it’s a part of random damage. Random Damage will deal damage from a range, the lower number is equal to half of the spell damage (the higher number is the normal spell damage amount). This also applies to any boosts that affect the spell.
eg. I have a Bullserker with 27 Magic. If his spell was a normal damage, it would deal 3 base damage + 27 Magic = 30 spell damage. However with random damage it is displayed as 15 - 30 damage.
This happens because random damage will take half the normal damage as the lower half of the range, while the normal damage is the top half of the range.
When random damage is boosted, the same thing occurs. My Bullseker’s boost is +52, this means the normal damage would be 82, making the damage range on the random damage to be 41 - 82 damage. (as the lower part of the dmg range is halved).