Your guess is exactly right.
The merge syntax you are using - ActivityId={{$Model.All_Events.data.0.Id}} is what we call “Global” syntax. What is nice about this method is that you can use it wherever we process merges throughout the application. The problem is that you don’t have “context”. That merge will always retrieve the Id field from the FIRST row of the “All_Events” model. (The 0 index). You get the same event every time…
Any data that is connected to a model is going to give you “row context” If the calendar events in your calendar are bound to the All_Events model - and you simply use the field in braces like this — {{Id}} then the Id field will be specific to the row of the event you clicked on.
You can test this by simply putting a template in the popup with {{Id}} to make sure different IDs are displayed for each event.
