use a Lerp instead of adding it. Timelines are not guaranteed to tick every frame so your results are actually always different.
Use a linear curve in your Timeline that goes from 0 to 1. In the Update, you simple use a Lerp. Into A, you put your current HP, while in B you put your desired HP.
Make sure that you subtract damage taken etc from Desired HP, otherwise Damage has no effect on it.