Hello, is this using an OOTB operator or in your custom app ? I just did a test in our 7.22 system, granted it's using Pega 7.22 as well and not 7.3. I created an interaction, which opened in a new tab. Then, in that interaction, I created a service process. It opened in the same interaction tab. Then, I went to the home tab and to "My Cases" gadget and opened some other process. That opened in it's own tab, not within the interaction or home tab. The only time the work from the list would open within the interaction tab is if you are trying to open the same work item that's already open in the interaction. At that point, you would get an error as well saying that the item is already open and this action will cause you to lose your changes. 

Please reply with a step by step process of what you are doing and if this is OOTB or in your own application. 

thanks