leading 3 animals on leads simultaneously can sometimes result in one animal vanishing.
Reproduction:
1. create 3 Leads
2. target 3 creatures, attach leads.
3. traverse distance with them. Note that one creature eventually vanishes; the lead that it was attached to may or may not be left behind.

in my specific example, the 3 creatures used were donkeys with loaded saddle packs. This first occurred after ~2000 blocks had been traversed in an east direction, then after the first one vanished, a wolf was tamed and leashed in addition to the two remaining donkeys. after another 3-400 blocks were traversed in an east direction, a second donkey was lost.