Many thanks @aleix - it turned out we had left behind some rows in the group_content table that referenced items that were (at one point) found in the group_content__entity_id_str table to support the essentially defunct groupmenu module. We removed references to those items in both group_content_field_data and group_content tables and we stopped having this issue. (We have moved away from groupmenu and are adopting group_content_menu - but getting out from under the patch from https://www.drupal.org/project/group/issues/2797793 ✨ Entities identified by strings as group content Closed: won't fix was a bear.)
@aleix - I realize your comment at #3 is five months old, but do you recall how you identified the 'faulty records' in tables group_content_field_data and group_content that needed removing?