This is just based off my own experience when they don’t show, but if you haven’t tried it then move them around a bit in the 5 spots they can be placed in the pen tends to work for me, when they stop showing for whatever reason
This is not the case when I play on our Xbox - Rented Server. Each adult pet can be seen in one section of the pen. It is actually pretty cool, because they growl and make other noises as caged animals would.