I didn’t notice any health returns at 3%, 6% or even 9% so I checked with and without to see if maybe I was getting the health instantly on cast and just wasn’t realizing. But no, unfortunately it’s not that.
With Siphon(any tier): with 125 hp, Transplant drops me to 100 health, no health regained from walking around.
Without Siphon: with 125 hp, Transplant drops me to 100 health.
So, definitely not working.