Inactive 'Prompt' checkbox in Updatable Dashboard

After some more playing around, and keyword searches, I landed on this prior post where you chimed in. This is what I’m experiencing: the field I want to update is in Subquery 2, with Left Join.

Oddly, within the uBAQ I can update the field. However, the Dashboard will not allow me to select the field as a Prompt. For further testing, I chose a field from Subquery 1 to be updatable, went back into the dashboard, and voila! I was able to select that top-level field as a Prompt.

At this moment, I definitely feel comfortable thinking it’s because of the subquery. I’m a newb with advanced coding so I’m not real sure how this is going to play out.