It has been found in the Reports (Insights) that the Exchange Online Hybrid User Configuration gathering will overwrite the custom attributes from the corresponding hybrid user's on-premise Exchange mailbox, with any custom attributes that are set on their Exchange Online mailbox.
The Custom attributes that were written the Exchange Online Mailbox are then reported for the on premise custom attributes in the UCA insights.
Custom attributes that are created for users on-premise mailboxes, are being overwritten after the Exchange Online Hybrid User Configuration runs.
Be mindful that custom attributes written to the mailbox in Office 365 will be replicated (overwritten) on the On-premise mailbox custom attributes and then will be reported in the Insights. At the time of this writing (8.5 Hotfix 3), any Custom Attributes changed in the cloud the administrator should expect to see them on their on-premise mailbox as well in the UCA Insights.
STATUS
Waiting for a fix in a future release of Unified Communications Analytics
