There are two parts to this skill: the active cast, which costs health in return for mag and stam; and the passive bonus for slotting it, regardless of whether it's ever cast, which is what returns health on any damage done.
Soulcleaver takes ultimate for each time you cast the skill, and it correspondingly reduces the health cost to cast it. But it does not take any ultimate for the passive heal from this skill, and so it cannot give its bonus to that part. (It likewise will not actually reduce the cost or improve the healing of any skill cast when you have 0 ultimate, as it can't exact its price.)
If it were to increase the healing done, it would also have to drain you of 1 ultimate every time the healing passively procced.
ETA: To clarify, I'm not actually advocating that Leeching should drain ultimate every time the healing procs. With HoTs, the entire healing over time is included in the budget of casting the skill, so each tick should be buffed when Soulcleaver applies. With Leeching, however, the passive healing proc (which is distinct from a HoT) is not included in the budget of casting the skill at all. It exists whether or not you ever use the skill. Since that's the case, the price taken in ultimate for casting the skill doesn't seem to me like it should apply to the always-on passive heal proc, and so the set bonus shouldn't either.
While in combat, strengthens your Siphoning abilities at the cost of Ultimate:
Increases the damage and healing of Siphoning abilities by 18%. Reduces the cost of Siphoning abilities by 18%. Casting Siphoning abilities drains you of 1 Ultimate.
There are two parts to this skill: the active cast, which costs health in return for mag and stam; and the passive bonus for slotting it, regardless of whether it's ever cast, which is what returns health on any damage done.
Soulcleaver takes ultimate for each time you cast the skill, and it correspondingly reduces the health cost to cast it. But it does not take any ultimate for the passive heal from this skill, and so it cannot give its bonus to that part. (It likewise will not actually reduce the cost or improve the healing of any skill cast when you have 0 ultimate, as it can't exact its price.)
If it were to increase the healing done, it would also have to drain you of 1 ultimate every time the healing passively procced.
ETA: To clarify, I'm not actually advocating that Leeching should drain ultimate every time the healing procs. With HoTs, the entire healing over time is included in the budget of casting the skill, so each tick should be buffed when Soulcleaver applies. With Leeching, however, the passive healing proc (which is distinct from a HoT) is not included in the budget of casting the skill at all. It exists whether or not you ever use the skill. Since that's the case, the price taken in ultimate for casting the skill doesn't seem to me like it should apply to the always-on passive heal proc, and so the set bonus shouldn't either.
Except this is not how the set works at all - the set is simply supposed to give 18% more damage/healing and cost reduction on Siphoning Abilities as long as you're in combat, whether you drain ultimate or not is irrelevant.
You can test this by casting vampire ultimate for example and being stuck at 0 ultimate - your tooltip will go up by 18% the moment you enter combat.
Also other passive effects such as the damage over time of Tether & Crippling Grasp gain +18% damage without consuming any ultimate, as it is supposed to work.
Now, if you cast Leeching Strikes or Siphoning Attacks for the resource return, you will drain 1 ultimate each time (if you're in combat) as they are Siphoning abilities.
All in accordance to the tooltip:While in combat, strengthens your Siphoning abilities at the cost of Ultimate:
Increases the damage and healing of Siphoning abilities by 18%. Reduces the cost of Siphoning abilities by 18%. Casting Siphoning abilities drains you of 1 Ultimate.
There is no reason why the healing of Siphoning Attacks/Leeching Strikes shouldn't also benefit from this set, whether it is passive or not.
There are two parts to this skill: the active cast, which costs health in return for mag and stam; and the passive bonus for slotting it, regardless of whether it's ever cast, which is what returns health on any damage done.
Soulcleaver takes ultimate for each time you cast the skill, and it correspondingly reduces the health cost to cast it. But it does not take any ultimate for the passive heal from this skill, and so it cannot give its bonus to that part. (It likewise will not actually reduce the cost or improve the healing of any skill cast when you have 0 ultimate, as it can't exact its price.)
If it were to increase the healing done, it would also have to drain you of 1 ultimate every time the healing passively procced.
ETA: To clarify, I'm not actually advocating that Leeching should drain ultimate every time the healing procs. With HoTs, the entire healing over time is included in the budget of casting the skill, so each tick should be buffed when Soulcleaver applies. With Leeching, however, the passive healing proc (which is distinct from a HoT) is not included in the budget of casting the skill at all. It exists whether or not you ever use the skill. Since that's the case, the price taken in ultimate for casting the skill doesn't seem to me like it should apply to the always-on passive heal proc, and so the set bonus shouldn't either.
Except this is not how the set works at all - the set is simply supposed to give 18% more damage/healing and cost reduction on Siphoning Abilities as long as you're in combat, whether you drain ultimate or not is irrelevant.
You can test this by casting vampire ultimate for example and being stuck at 0 ultimate - your tooltip will go up by 18% the moment you enter combat.
Also other passive effects such as the damage over time of Tether & Crippling Grasp gain +18% damage without consuming any ultimate, as it is supposed to work.
Now, if you cast Leeching Strikes or Siphoning Attacks for the resource return, you will drain 1 ultimate each time (if you're in combat) as they are Siphoning abilities.
All in accordance to the tooltip:While in combat, strengthens your Siphoning abilities at the cost of Ultimate:
Increases the damage and healing of Siphoning abilities by 18%. Reduces the cost of Siphoning abilities by 18%. Casting Siphoning abilities drains you of 1 Ultimate.
There is no reason why the healing of Siphoning Attacks/Leeching Strikes shouldn't also benefit from this set, whether it is passive or not.
As you quoted, the tooltip literally says it requires paying "the cost of ultimate" in order to give its benefit, and that's how it's worked previously. If it's giving you the benefit (tested in cmx, not just relying on a tooltip?) without being able to drain ultimate, as in your vampire example, that would seem to be the bug to me.
DoTs and HoTs are also not passive. They are part of the budget of casting that skill actively. Therefore they properly benefit from Soulcleaver as long as ultimate was taken when their skills were cast.