Weird Dog bug where if my dog's hp is full, and then if i re-enter world, it's hp is very low

This is weird, I was playing minecraft and tamed a lot of dogs and I healed them to full hp. After a while of playing, I needed to leave the world, but when I came back, their tails were down, all of them. Even if I healed them, their hp after leaving and joining the world was low and the food items I used on them was gone. It must be a glitch or something.