The information on this bug is available in this thread.
https://boards.na.leagueoflegends.com/en/c/bug-report/44RI0624-gameplay-sion-will-not-receive-health-from-large-monstersminions-if-w-is-leveled-up-in-passive
Only incorrect information is the amount of health received from minions and champion kills.
Copy and pasted from the link given above.
If you level up Soul Furnace (W) for the first time while in Glory of Death (P), Sion will be unable to receive 10 bonus health from all large monsters/minions. To encounter this bug, you must not level Soul Furnace (W) if Sion is charging in his unplayable form (where you cannot attack/move), only level it up when he can attack and move in that form. He is still capable of receiving 10 bonus health from champions and 3 bonus health from any other units.
#Instructions:
- Play at any level as Sion, when you do not have Soul Furnace (W) leveled up at all.
- You must die to enter Glory of Death (P).
- Once you see you are able to move, attack, or use your movement-boost ability in your new form, level up Soul Furnace (W) before your health deteriorates to 0 HP.
- You will no longer receive bonus health from large monsters and large minions.
This happens 100% of the time if performed using the steps in order.
-Bug Catcher