When a date has a start date and end date, it's no longer two values in the date field and has been instead moved to the daterange field type. Date popup should work on this field type.
I believe it's related somewhat to this patch.
https://www.drupal.org/project/date_popup/issues/2983680 →
Add a daterange field to a node. Expose the date range field in views. You won't get a date popup and you'll instead have a plain text field.
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