Hostname/Device ID cache seems wrong in Syslog when re-using a hostname

We replaced some equipment lastly, but there is a period of time when both the old one and the new one have to be functional.
So we renamed "hostname" in "hostname-old" and added the new device under the name "hostname".

Problem : now the logs collected from "hostname" (the new) go under "hostname-old", mixing with the legitimate "hostname-old" logs.

Syslog sets the $HOST variable correctly : I also write all the logs to a plain text file and these are fine.

I see in includes/syslog.php that there is a cache of the association hostname-device ID (extract from line 27) :

  $now = time();

  $expired = TRUE;

  if (isset($dev_cache[$host]['lastchecked']))

  {

    if (($dev_cache[$host]['lastchecked'] - $now) < 21600) { $expired = FALSE; } // will expire after 6 hrs

  }

  if ($expired) { $dev_cache[$host]['lastchecked'] = $now; }

But the timestamp in $now will always be superior to $dev_cache[$host]['lastchecked'], so the difference will always be negative, so $expired will always be set to false. That may be why all logs from the new "hostname" go under the device that was formerly named that way.

I'm not 100% sure this is the cause, I did not dig any further..