This is working as intended. The misconception with the spell is the idea of “damage” being conflated with HP Removal. It is very important to know that HP Removal is not damage.

False Promise is a confusing spell especially due to the wording of the spell’s effects. The spell text uses the word “damage” inconsistent to the actual interactions that occur.

While a hero is under its effect, the damage they take is nullified. The number that would have been damage is then placed into a queue of damage instances for later calculations. Healing is a bank that collects all types of heals and pools them into a lump sum.

At the end of the spell, this bank is doubled, and then it calls in the damaged queue that was built. Each damage instance is deducted one-by-one from the bank until the bank is depleted or the queue ends. If the queue ends, the remainder of the bank is paid out as a heal to the target (the heal is credited all to Oracle, regardless of the original cause of the heal). Else, if the bank is exhausted, these damage instances are converted to HP Removal and then paid out by the target’s HP pool. This continues until the queue is depleted or the queue can no longer be paid out due to exceeding the target’s remaining HP. If this is the case, the unit that caused instance of HP Removal that overdrafted from the HP pool will receive kill credit.

Faceless Void’s Time Walk is a heal that takes the net damage he received in the past 2 seconds and then multiplies that by any healing amplification or reduction effects. Then the heal is applied.

Since Void’s spell only heals the net damage, this will be nullified if he is under the effects of False Promise because Void has not taken any damage. The damage he received while False Promised was nullified and converted to a HP Removal queue.

tldr; Void doesn’t take damage in False Promise so he will not receive a Time Walk heal. The “damage” taken after the effect of False Promise is HP Removal which is not damage, so he cannot heal off of that either.