If you encounter this issue or a similar behavior, there are a few things you might want to check.
If the assignment rule fails to locate an owner then the lead will be assigned to the default owner as mentioned in the Salesforce setting or If you do not use assignment rules, all leads will be assigned to the Default Lead Creator.
- Lead ownership could be changed by Lead Assignment Rules, make sure to check these and compare them against the affected records.
- If no Lead Assignment Rule has been created, take into account that Salesforce has a default Lead Assignment Rule. Under Setup/Customize/Leads/Lead Settings, you will find the default lead owner. If a user edits a lead record and checks the "assign using active assignment rules" checkbox, Salesforce will evaluate all assignment rules and assign accordingly. If no assignment rule is found, Salesforce will assign this lead to the default lead owner.