Database Mail has incorrect encoding of CSV attachment
I would suggest you to generate the attachment with powershell and then send the attachment:
$reportQuery = &quot;select * from ##Report&quot;
$filePath = &quot;\\aaaa\bbbb\attach.csv&quot;
$mailQuery = &quot;
  EXEC msdb..sp_send_dbmail
  @profile_name = '..',
  @recipients = '..',
  @subject = '..',
  @file_attachments = '&quot; + $filePath + &quot;'&quot;,
    ..... (others parameters)

Invoke-Sqlcmd -query $reportQuery | export-csv $filePath -encoding &quot;unicode&quot;
$result = Invoke-Sqlcmd -query $mailQuery