Currently, the way it is coded, only waypoints that are [u]prior[u/] to the last SID waypoint are labelled with <is_sid_star>1</is_sid_star>. So in the example flight, since EPL is the last waypoint of the SID, it’s normal for it to not have the tag.
Whether or not this is “correct” is up for debate, but the XML data file has been this way for years now, so I’d be hesitant to change it in case it breaks existing implementations.
I understand what you say. The problem in case of e.g. EPL6T is, that EPL is the only waypoint on this SID. That makes it a bit difficult to find the SID in the XML Datafile. Of course, the SID is named several times in the datafile but more complex to find. Adding additional xml tags for the used SID / STAR should not break backward compatibility.