Hi Guys, just in case if someone still have this problem: Filter by category not working only if you used default filter/search, just click Catalog > Products to refresh and it will work. Explanation why it happens: After default filter/search used, #product added to the URL and then when you try to filter by category javascript adds your &id_category=100 AFTER #product which is the culprit why page is not refreshed with your selected category.
example: http://domain.com/backend/index.php?controller=AdminProducts&token=000000000000000000#product&id_category=100