Can't delete topic without category

Product Support
6

This seems not to work:

discourse(prod)> Topic.where(category_id: 4).update_all(category_id: 20)
=> 0
discourse(prod)> Topic.where(id: 3).update_all(category_id: 20)
=> 0

And in Data Explorer I get:

Select id, closed, archived, title,category_id
from topics
where id = '3'

result:
id 	closed 	archived 	title 	category_id
3 	true 	true 	Über die Kategorie Team 	4