I found a little bug related to this fix - according to wowhead comments, if Drain Soul is casted on a target below or at 25% HP, the damage should be still 4 times higher even if the target gets during the channneling over 25% of its HP (f.e. it is healed by somebody...).
Currently, the damage is 4x higher if at the time of the periodic tick is target below or at 25% HP.
well first of all drain soul is not only spell that is affected by this bug, there are a lot more spells. The problem is that the dmg should be calculated at the moment of casting, but currently it is calculated for every tick. Same problem is with lava burst for example, many ele shamans exploit it, they first cast lava burst and right after that they cast flame shock, so when the spell hits target it is crit, but again the dmg should be calculated at the moment of casting not at the moment of impact...