From Catalog > Manage Categories I select a category and go to "Category Products" tab.
I reset filters and select one or more products to add to category. Then hit "Save Category" button and the category is indeed saved but the products have not been added to the category.
If I remove a product from a category then it is saved and removed correctly.
I have tested with new and different categories and the problem is the same. I have cleared caches, checked logs and there are no errors neither exceptions, no js errors as well.
Any possible idea would be appreciated.