Mystery of  Calendar Permission role - PowerShell - Reviewer

I'm struggeling to handle roles for calendar...
The purpose is to open calendar of people who leave the cy.
I'm using this :
Try{Add-MailboxFolderPermission -Identity ${$CalendarDestinationAlias}:\"Calendar" -User $UserToGiveAccessToAlias -AccessRights $Rights -erroraction SilentlyContinue}
Catch{$bOk = $False}
Try{Set-MailboxFolderPermission -Identity ${$CalendarDestinationAlias}:\"Calendar" -User $UserToGiveAccessToAlias -AccessRights $Rights -erroraction SilentlyContinue}
Catch{$bOk = $False}

where Rights is "Reviewer".

Unfortunately even with Reviewer, I only see free/busy information but not the full details of the agenda....
WHY????
Do I need first to be sure that user $UserToGiveAccessToAlias has NO Access at all ? (if yes then how?)