Passive mobs dissapear when going thru portal

Just a few days ago I was playing with my sister, she was trying to get a cat, and the fastest way to get back home was using a nether portal. So I made the portal and when the cat went into it, it never appeared on the other side, and I know it didn't die 'cause there was no message in chat as usual.

The same thing happened today, but this time it was 7 parrots! Same, I pushed them into the portal the first time and only 2 appeared at the nether, then inside the nether when I finally arrived at my home portal with both birds and again pushed them into it, they never appeared on the overworld. 

But I have to say that it's pretty inconsistent, 'cause I was able to get back home with a strider, 2 pandas, and 1 blue parrot, but still, it's very annoying just to lose them without doing anything wrong