Mobs Dissapearing

I was in survival and I found two dogs, so I dug a hole for one, a normal one, and the other a puppy, I went back to my base which is only a couple hundred blocks away to get bones and when I came back both dogs disappeared.  They didn't escape, that would be impossible since both holes were two blocks high.

Steps to reproduce

Spawn in two dogs, or find them naturally

If spawned in then switch to Survival

Dig a two block deep hole for each dog

Go about 200-300 blocks away, wait around 15-20 seconds and then return to the dogs

They should not be there