Well, when you come back after walking away from where you left your pets, they just disappear, not even a message comes out that something happened to them. (They are tagged)
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Thank you for your report!
We're actually already tracking this issue at MCPE-47922, so we will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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I think your pets may be on chunk borders which as right now are bugged and will not properly save mobs once exiting the world or possibly unloading said chunk.