Slayer overleech not working with instant leech

I don't know if I found a bug or I'm unaware of some mechanics but Slayer overleech is not working with instant leech.
I'm running strength of blood so, non- instant leech is not appiled. But I've instant leech from mastery and from charm and it is healing me. I also have slayer overleech but it's not working. For example, when I kill a pack of monsters, and I will stand on burning ground nah hp is going down, but with brutal fervour it should stay full for a while.
Everything was working fine when I have both normal and instant leech.
So I don't know if I don't understand something or if I found a bug.
Thank you in advance for your answer!

Non-instant life leech is not applied, it is instead transformed into damage reduction. So it is correct that you will dip health while walking over burning ground and such if you don't have passive life regen that covers for the HP loss.

The way instant leech interacts with this is that let's say you have 20% instant leech, now only 80% of your life leech will be transformed to damage reduction. The 20% will be applied instantly, and anything above max health is lost.

So you want to balance between how much instant leech is enough to maximize the damage reduction

Thank you for your answer!
I've Vaal pact, so iI've no regen.
I've one more question. How Brutal Fervour interacts with that?
"Life Leech effects are not removed when Unreserved Life is Filled" it should mean, that life leech effects are not removed even if leech is instant. But in my case of my build as soon as I reach full life leech effects are removed.

So you have to think about life leech like a Health Globe, much like your character's health pool. It scales directly with your maximum HP and modifiers such as "Maximum total Life Recovery is doubled" from Vaal Pact. You fill this separate health pool up by leeching life into, which then gets depleted over time. Brutal Fervor just makes it so that when your health reaches full, life leech effects (the separate pool) isn't emptied to 0.

Instant life leech works differently from this leech pool. Basically you take a chunk of your regular life leech away and turn it into an instant heal on every attack. It is thus unaffected by the Slayer overleech and cannot be considered for either the overleech or the damage reduction from the keystone

Btw, with Replica Farruls Fur I am surprised you didn't take Masterful Form to get an extra Frenzy Charge and then set your Endurance Charges to your max frenzies. It's a ton of physical damage reduction and also resistances you have up permenantly, freeing up more affixes on gear

I did not know that! Thank You!
I didn't play for something like 2 years, so I'm still learning what was changed.
I was thinking about Masterful Form but I also needed it accuracy. But now when my idea with leech didn't quite work out I should rethink few ideas.

Leech has always been an "over time" effect, much like taking a potion. with the exception of old school vaal pact and LGoH (which is NOT leech), this has always been true since the beginning of the game.

Instant is....instant. Everything is applied instantly and thus couldn't possibly carry over into anything that happens after that instant.