I’m not sure about what you’re seeing or that other post, but I have a repeater I just tested with and with or without, it also depends on the layout option set for the repeater. Displaying as a row, you can collapse the repeater even without making a selection. It is when you choose the table or block layout that you must select a field to show when the repeater is collapsed. It could be a bug, I don’t know. If you want to report this as a bug you should submit a new support ticket https://support.advancedcustomfields.com/new-ticket/
