Definitely understandable!
The relationship model in DHIS2 is very flexible, TEIs, enrollments, and events can all be linked in the backend (Through relationship types configured in the Maintenance app, which is what you mean by “the UI allows you” right?) but we have not yet implemented all the options in our capture app UIs. So I see how this can cause some confusion. The docs include for the Capture app " Currently the Capture App only supports Event to Tracked Entity Instance relationships." (Which will also need to be updated soon :))
As you have gathered from JIRA issue 8214 (and the fact that it does not work in Tracker Capture), we have wondered if relationships meet the requirements that the Tracker Associate value type does. Tagging @Markus and @Stian as well for this particular topic. We wrote a post on the community a couple of years back to gather a bit more around the use of Tracker Associate as well: Remove tracker associate data type
Just to clarify, you would want to link many TEIs to an event in a program stage in a Tracker program, yes? Can you describe the usecase a bit? The same thing wouldn’t work if they were single events in an event program, right?
Best,
Karoline