Can’t empty recoverable folder, it might be caused by DelayHoldApplied.
Delay Hold indicates that a temporary delay has been set for the removal of a Hold. When any Hold is removed from a mailbox, this value is set to True to prevent the accidental purge of content that’s no longer affected by the Hold. When DelayHoldApplied is set to true, the mailbox is still considered to be under a Hold.
Search-Mailbox -Identity email@example.com -SearchDumpsterOnly -DeleteContent