I reported this before and i lost my pets but luckily i got them back and now i saw the bug again and i wanted to pointed this one out since last time i didn't have any screenshots to post.
Picture one you can see the Pet incubus on the screen.
Picture two you can't see the Pet incubus but you can still open up its status bar
NOTE: When you open the status bar and you return it to an Egg you wont receive the egg and the pet will be totally be gone. This leads to the disappearance of pets