I’ve been doing some testing with healing items recently, both food and potions. I believe the ‘drop’ in health you noticed when eating is not a drop at all, its your maximum health increasing; which makes your current health appear to be lower. It seems to be that when you are 100/100 on hunger, and perhaps water, you receive an increase to your maximum health and stamina of around 5%.
This has actually been mildly annoying in my testing, since some healing effects seem to be percentage based, and some healing effects (such as food) increase your hunger, therefore increasing your max health, which in turn effects healing done. 