This caused by old information of this security group is stored in the site collection user information list.
1.Go to user information list of the site collection (https://tenant.sharepoint.com/sites/sitecollectionname/_layouts/15/people.aspx?MembershipGroupId=0) -> Select the DocApproval security group -> Actions -> Delete Users from Site Collection.
Note: You should be a site admin to access user information list.
2.When filling in the Person or Group Field of SharePoint list, select the Doc Approval security group.
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