pretty easy to reproduce:
1. create a new reseller, let's call him res1
2. login to res1.
3. create a client under res1, let's call it user1
4. logout res1 (back into admin)
5. go to admin backupt/restore
6. select the reseller.
7. start backup
8. delete reseller res1
9. confirm all res1 users will be deleted.
10. restore res1 from the backup you created in step7.
11. reseller res1 is back online (with all its websites/domains) without any user under it!
M