The Absorption Effect is given to the player every three second for just a very short amount of time, instead of refreshed every three seconds. This causes it to go permanently on and off, constantly changing the heart bar and the effects in the inventory, as shown here: https://i.imgur.com/r9phiX6.gifv

To fix this, it would be simpler to refresh the Absorption effect every so often (three seconds may be too short, so potentially scaling the refresh time depending on what level is the best way to go), and not remove it after a few ticks.