On a multiplayer server. To recreate, have two players, one with the breeding item of some animal (discovered with sheep and wheat), the other without, and attempt to lead the animals past the other player. When the other player is the nearest player, the animals will stop following any player, even if the other player is holding the breeding item in both or either hand and makes sure they didn't de-select the breeding item.

PLEASE NOTE this is different than the "issue" of animals that loose interest taking some time to gain it back, I realize that is an intended feature. This is a matter of animals loosing interest simply because another player is nearby.