Show empty groups with filtered lookup column


I use a lookup column (sub-cat) filtered by another column (cat). It works phenomenal.

To simplify the reading of the table, i group the entries by cat and sub-cat. And i would like to show the empty group. I hope a test docs could help to understand :

Unfortunately, it doesn’t only show the filtered sub-cat by cat as i did in the looking. But for all the cat, it shows all the sub-cat.

Any idea to get only the filtered sub-cat even if there are empty?

Thank you in advance for the help