to reproduce:

1. build a nether portal and get a tamed sitting cat/dog
2. push the cat/dog in the nether portal, so it goes to another dimension
3. go in the nether portal
4. click on the cat/dog, to let it stop sitting
5. You will see that only the nether portal block is selected, and not the cat/dog. You only can let the cat stop sitting by destroying the nether portal.

This bug also happens when the cat/dog is sitting in tall grass or flowers