Steps to reproduce:

1. Create three users in JIRA: "admin", "fred", "john"
2. Create a custom Multi User Picker field & add it to the default screen
3. Create a project if there isn't an existing one
4. Create an issue
5. In the multi user picker field, type "admin" and select him from the drop-down
6. ..., type "fred" and select him from the drop-down
7. ..., type "john" and select him from the drop-down

Result: you will notice that there are additional spaces being inserted in between "admin" and "fred". The more users you add, the more spaces get inserted.