Check to make sure the entity access rules for the role allow to write to the attribute. It may be possible the role only has read privileges. Also check if the dataview/listview editable field is set to 'Yes' in the General tab.
It doesn't appear from the data you provided that you've done anything incorrect on the configuration and setup so like you I would be scratching my head, especially since we can't recreate the issue. When troubleshooting I sometimes create a fresh example with a different association, no security, to remove that variable entirely first. Could you give that a go where you setup a similar one to many entity relationship entity model with either one security role that has all read/write in your model if in production mode or turned off and try the widget again to see if your project has the same behavior? That will help rule out the domain model as a cause and then we can focus on the page setup for help or the widget itself as a potential issue.
Thanks everyone for the response.Issue is resolved .Looks like it has something to do with the microflow I used for retrieving items. when I changed it to a simple one its working fine. Didn't think of its microflow issue as it was working fine with Simple checkbox set selector.