I did a test with the command you provided, and the result is the same.
After double checked this command, i think it is because of the parameter:
(received:(<01/01/2018) OR sent:(<01/01/2018))
Seems like the new version Exchange didn't support this format.
Try using this command and test:
Get-mailbox -Identity email@example.com | Search-Mailbox -SearchQuery 'Kind:Email AND Sent<01/01/2018 OR Received<01/01/2018' -DeleteContent -force
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