There’s no (sub)category “Bracket, Decorated,” so they show along the plain ones.
Conclusion: for consistency there should be a "Bracket, Decorated" subcategory
... now that there are
They don't automatically get a patterned category. There are lots of patterned
parts in other categories that are not named 'decorated'
Stops the tree page being even bigger than it already is.
I would say that for now we are probably being cautious to add more to the tree.
As the site gets updated I would imagine decorated categories could become more
commonplace even if there are only a few patterned entries in them. We will have
to see how things develop.