Healing using Rotten Hearts triggers the stat boosts for Candy Heart.
This item does not give stats when adding to Tainted Bethany's blood charges.
This item is useless for characters that can't have red hearts (e.g. The Lost, Keeper, ???, etc.).
Any healing prior to picking up Candy Heart will not provide stat boosts.
The stat bonuses are "stored" on the Candy Heart. Dropping or losing it in any way won't immediately revert the bonuses gained, only when Isaac's stats are updated again. The respective stat changed is the one that loses the Candy Heart bonuses.
The bonus stats will be lost if Isaac's stats are updated by dropping out of the game or if the game crashes.
The stat bonuses are seeded, meaning the same stat ups will apply in the same order in the same seed.