When you are killed by enemy who has learned the Essence Shift, you will get EXTRA regeneration, base on your items in your inventory.
In this gif ↑, the Windrunner has +25.6 regeneration before she died, but it become +50.7 when she respawn (the hero get extra & permenant +25 regeneration from Ring of Health*5).
These extra regeneration can be stacked, when you triggered this Bug.
In this gif ↑,the Axe have +103.7 regeneration without any item and skill.
Just because he was killed by the enemy 4 times while trigger this Bug. (5 items*5 regeneration*4times=100)
Attached please find the Regeneration Bug with Essence Shift.w3g
Another skill which has the same effect is Spit.
In this gif ↑, the panda is also become +51.4 regeneration after he respawn.
You can also find the attached Regeneration Bug with Spit.w3g to check it.
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The point of this BUG, i guess, is
when your attributes (which have stolen by the enemy) return to you after a period of time (Essence Shift 20s & Spit 12s), while you are die in the heaven, then you will get all the regeration from your items in your inventory.