When you bond a beefalo with a bell, it still remembers its herd. You need to either log off and back on with the bell in your inventory, or go to the caves with the bell in your inventory for it to forget its herd.
Also, when a beef *does* forget it's prior herd, it's heat timer is reset. Of course, because beefalo mood is tied to the beefalo herd. I'm not sure if the heat timer being reset is intended or not.
1. Find a beefalo in a herd
2. Bond with the beefalo
3. Go far away
4. Notice the bonded beefalo still tries to walk back in the direction of it's herd
5. Relog or switch shards with the bell in your inventory
6. Notice the beefalo now forgets its herd