Thank you for writing.
The observed behavior is expected and it follows the pivot behavior in excel which is the same. The items building the tree of the filter popup are populated according to the distinct values provided by the group descriptor. Once a filter gets applied to the group descriptor the popup is updated to display only the items passing the filter.
Applying a filter on a different level does not automatically update the nodes collection of the other group descriptors. Hiding certain nodes from a popup of a descriptor element which is a subcategory of another descriptor element is possible. For the purpose, you would need to store the unique filtered items of the parent category and once the filter popup of the subcategory is shown, hide nodes according to the filter. Please not that it is not a simple task and may not cover all possible cases.
I am sending you a sample project showing a sample implementation as well as a short animation showing the result on my end.
I hope this helps. Should you have further questions please do not hesitate to write back.
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