Tasklist filter with JPA or JSON variables

When I edit my task list filters, I can add variables that will show up together with the tasks. This works very well with strings, numbers, and other scalar process variables.

However, I cannot use objects that are persisted with JSON or JPA/Hibernate.

For example, one of my process variables is an object called “trial”. I am interested in “trial.subject.username”, but the result is simply empty or

I can’t find the specific documentation at the moment, but as far as I know only primitive variables are available for filtering due to the fact how complex objects are stored.