"is not" operator for Subproject is expected to exclude only given projects, however, it actually excludes closed projects as well.

Suppose the following project tree.

parent(status: active) -- issue15
 +-- child1 (status: closed) -- issue16
 +-- child2 (status: active) -- issue17
 +-- child3 (status: active) -- issue18

When you apply the issues filter "[Subproject] [is not] [child3]" on the "parent", issues from "child1" and "child2" should be displayed but you will see issues only from child2.

Applying issues filter "[Subproject] [is not] [child3]" on the "parent":

[Expected result]
issue15, issue16, issue17

[Actual result]
issue15, issue17

The behavior is clearly inconsistent with the "is" and "any" operator, because the "is" operator is can be used for closed projects and displays its issues, and the "any" operator includes closed projects as well as active projects.