Life Leech bug with Soul Tether belt

I tried making a build with Soul Tether belt, which has a line:
"Excess Life Recovery from Leech is applied to Energy Shield"
I would expect it to apply all life leech to ES when I am on full life, but that's not what actually happens.
It seems like when you have full life, the leech effect doesn't trigger at all.
From my testing it seems like this:
so if you have 1 life missing, and getting 40 life leech, it would apply 1 to life and 39 to ES.
but if you have 0 life missing, you would not get any ES leech.