[Gameplay] Executing with multiple on-hit magic damage miscalculates spellvamp

In patch 7.7 I saw this bug report where the OP said that Lichbane was bugged, causing Spell vamp to not apply to overkill damage. I reckon all on-hit magic damage suffers from this (hard to verify). 2 patches later, 7.9, this is still not fixed, nor have we received any notice of Riot confirming this.

It comes down to this: when you execute something with 2 sources of on-hit magic damage while 1 both of the sources can be fully overkill damage (damage that was not needed for the execute), both sources will go on cooldown, but do not both apply spellvamp.

The bug can be reproduced effortlessly. here are the steps to reproduce it:

1. Pick a champion with on-hit magic damage, preferably with a lot.
2. Make sure to get a second on-hit magic damage source.
3. Get some spellvamp
4. Don't have full HP
5. Execute a monster such that their HP is lower than the lowest on-hit magic damage
6. Look at the healing from spellvamp

The healing from spellvamp is lower than what you expect if you calculated it.

This is a mathematical replication of the situations I used, where AA = basic attack damage, HP = Health target B = Lich Bane damage, C = Kassadin's Nether Blade damage and C > B HP > AA + B + C --> All damage is applied, nothing special happens AA + C < HP < AA + B + C --> All damage is applied, overkill damage is also transformed into health AA + B < HP < AA + C --> Only B is redundant, but spellvamp over AA + B + C is calculated (is that fixed?) AA < HP < AA + B --> Either B or C is redundant, in this situation, only AA + C happens to be calculated HP < AA --> Either B or C is redundant, in this situation, only AA + C happens to be calculated

This phenomena is not seen for physical on-hit effects (tested with Aatrox with trinity force).