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.

Hi Grant,

Try this: in Admin/Setting/AdminSearch change the value of "Category size limit" to at least the current total number of active categories you have. By default I think it is set to 100, so if you have more than 100 categories the dropdown lists will not be able to populate all of the categories. The dropdown list and one other place in the Vendor module are the only places that currently use that parameter as the default. If you have under 100 categories, this will probably not fix the issue you are having.

Hi Jim,
That was it. I find it strange to start limiting category qty especially without notification.
Anyway all good now.
Many Thanks

Grant