When at max hunger with less than 6 saturation left, the healing slows down and does not use all of the possible saturation.
This causes it to take about 5 seconds to heal the last half-heart from saturation, which is longer than the amount of time it takes to heal without saturation or when not at full hunger.
The fact that the saturation amount used for healing is capped at 6 and it gives you 6 exhaustion when you heal makes me think that this is a bug. Since 4 exhaustion removes one saturation, it should divide the player's saturation by 1.5 instead of by 6 when calculating healing.
Please provide consistent and clear steps to reproduce the described behavior.