The Status Waiting for Requester is an OnHold status and transitions through "Resume Work" to the Open status.

The "Resume Work" makes a check, if the Technician is empty, it sets it to Logged in user

The trouble we're now having is that if the Requester answers by e-mail, Servicedesk Plus will  transition the ticket to Open anyway. When we then try to assign the  ticket to a Support Group (unassigned) it triggers the transition rule for "Resume Work", detects that the technician field is empty and then sets the technician to the logged in user.

Why does the transition rule trigger when we update the ticket in the Open state? The transtition has already happened.

This was an issue in Request Life Cycle, and it's still an issue in Request Workflow.