Steps to reproduce the issue

1. Create an user with create + edit permissions for articles
2. Edit(!) an existing article
3. While you have open the article view form, change the permission of this user and remove the "create" permission
4. Create a new category onthefly (typing in a new name in the category select and confirm with [ENTER])
5. Save article

Expected result

An error, that creating an category is not allowed

Actual result

Article is broken (no category assigned)

System information (as much as possible)

Latest Joomla! 3.8 and Joomla! 4.0

Additional comments

In fact as coder, you could avoid the permission step, just create a user without create but with edit permissions and change the value of the category to a "name" like "my new category" (in the category select)