I created a custom item with a attribute of +20 health (double the health). When held long enough the meter fills up, however, when I deselected the item the health did not update. Although the heart display updates the actual player health does not.

I detected this issue by using a testfor command to look at the players health. After deselecting the item the comparator remained on even though it was not supposed to. This did not change until the health was updated (taking damage).