Timers are running on server that is not configured to do that

We expanded our webfarm with a second server for internal use. So we have now two internal servers and one in DMZ. In ServiceCenter we configured the new internal server to execute timers. The second server started to execute timers. To our surprise the DMZ server also started to execute timers (and process Emails). After changing back the configuration to the earlier state the second internal server stopped executing timers but the DMZ server continued executing timers and processing emails.
Any idea’s why this is happening?

Version 9.01.19 , On-premises

Hello Gert. Is this still happening? Can you please contact our support on that if its still going?