I published the service from ArcMap 10.7.1 with editing enabled. Before I had a chance to test the date field solution proposed by Previn Wong, I stumbled into different solution for the smarteditor widget; while troubleshooting a separate issue for a URL not displaying properly in popups, I deleted the text field containing the URL. This was done in AGO. Next, I opened the app containing the Smart Editor widget and the configuration dialog recognized the hosted feature layer. To test this, I added the text field and URLs as they originally existed and the Smart Editor issue reappeared - it would not recognize the hosted feature layer for configuration.
I don't have a proposed solution, but there is something going on beyond the date field concern.
Another point worth mentioning is that I have attachments enabled on this layer. Also, due to privacy concerns, I am unable to share the app with the public.
Did anyone else who had (has) this problem happen to have a field containing URLs in their hosted feature layer?