During Exchange vNext coexistence, you can’t find a user who was migrated to Exchange vNext in People Picker in the Exchange Admin Center (EAC). Additionally, the "send as" and "send on behalf" permissions can’t be added for legacy dedicated or on-premises mailboxes.
After mailboxes are moved to from the legacy dedicated or on-premises environment to the Exchange vNext environment, they are represented as Remote Mailboxes. These remote mailboxes are not displayed in the EAC People Picker.
This issue is being investigated by Microsoft. To work around this issue, run the following Remote PowerShell cmdlets to add these permissions: