Hello 

I cannot reproduce the issue. I did the following:
1. Created an ad in 'Real estate for sale' subcategory of 'Housing & Rentals' category.
2. Removed the ad. Category is still showing that it has 1 item which cannot be seen during browsing. It's a bug: https://github.com/unaio/una/issues/2528
3.Then I go to Studio -> Ads -> Categories and remove 'Real estate for sale' without any problems. I even tried to remove the whole parent category 'Housing & Rentals' and did it without any problems too.

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Best Regards
Anton