Might be a bug. For example, you have first pull that contains 2 mobs, When you add another pull and choose the same mobs (feels strange that you can do that), if you delete the 2nd pull because its unnecessary, the first one bugs out and doesn't give any count. More than that, it just makes the initial pull empty.

1. add a pull
2. choose a few mobs
3. add 2nd pull
4. choose the same mobs
5. try deleting the 2nd pull