Can't filter by subcategory using a query

Hi all,
I have a list main categories and subcategories
CAT1
CAT2
SUB2.1
SUB2.2
CAT3

I filed several work items against "CAT2/SUB2.1" and I create a query with
the condition:
"filed against" + "is" : SUB2.1

But I have no results with this query.

What I'm doing wrong??

If I change the query filter to:

"filed against" + "in subtree" : CAT2

Is working as expected...

Any ideas?


Thanks!