Just upgraded to 2.3.0 and all sub categories have disappeared in the backend Parent category>selectcategory. When I re-link to the sub  category & save it. It keeps defaulting to select category in the parent category drop down list. If you make changes to sub category without re-linking it treats it as level 1 parent category.  Using IIS 10 MongoDB 6
Look forward to hearing from you soon, Grant.