The issue is in public function isParentCategoryAvailable($idShop) in Category Class. IMO it doesn't join the category correctly. I have 3 shops. (id_shop=3 is the default one). But in ps_category I have some entries, that have id_shop_default=1 (don't ask me how I can even controll this as an user).
In this case isParentCategoryAvailable($idShop) returns false, even if a category is associated to shop with id_shop=3.
https://github.com/thirtybees/thirtybees/blob/main/classes/Category.php#L2427
Should be changed to:
->join(Shop::addSqlAssociation('category', 'c', true, null, true))
This is a very edge case, which explains why I am probably the only one suffering from it 🙄