The Mailbox Inventory Insight is repporting an incorrect OU for some or all on-premise mailboxes. The Owner OU value that is listed is the Office 365 "OU" vs. the actual on-premise AD OU.
You have a Hybrid Office 365 Deployment, all Mailboxes exist in Exchange On-Premise with the Active Directory User Accounts being synchronized to Office 365.
The Owner OU should display the location where the account exists in Active Directory.
You are also running an Exchange Online Hybrid User Collection and a Domain Controller Collection.
Upgrade to Version 8.5.0 Hotfix 1 or later.