This issue occurs (to the best of my knowledge) when loading a chunk with a tamed cat or dog attached to a lead.
There are two possibilities of what may be going wrong:
1. Because of the tick processing order, the cat/dog loads before the lead does. These actions occur in the same tick but the cat/dog loading occurs first. If the player is far enough away from the pet, the pet may choose to teleport to the player before the game realizes the pet has a lead attached to it.
2. What I described above happens in reverse. The lead loads before the pet. It then realizes it isn't attached to anything, and pops off before the pet is loaded.
Of the two, it is more likely the first problem I described. I believe the second issue I described was fixed in 1.15.
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We're tracking this issue in MC-236289, so this ticket is being resolved and linked as a duplicate.
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