List filtering UI not working in "generating children from variable" feature

I'm reading list of Firebase documents using a backend query and trying to further filter them in related ListView (to create a simple kanban board where each column/list would be represented by a task state).

The condition form in filtering doesn't give field for the 2nd value, as can be seen from the screenshot.

If I try workaround by modifying the condition form so that the first value is a string (sort of) I can select as the second value all Document fields - except the ones with type String.

.. is this a bug in my brain, do I miss something ..

Thanks for helping!