Removing the Selected Item Transformation altogether makes no difference. HOWEVER ... I've stumbled upon the problem!
-- the asp.net Required Field validators!!! --
I had field validators to check required as well as valid email entry. The whole (small) form is displayed in a collapsible panel. Testing on a page with only 3 or 4 items listed, I opened all the panels, and could then easily see that submitting from only one of the forms triggered the validators for all of the forms! Just as an experiment, I entered data into all of the forms, and then submitted only one of them. Whalla! The form where I had clicked the submit button was the one that was actually submitted.