Text filter doesn’t seem to work

Hello,
I’m trying to setup a search form using WOOF (using the free version). I’ve enabled the “Searching by text” extension and have setup a text field, a custom taxonomy field and a categories field.

If I filter products using the text field only, WOOF doesn’t find any, even when using the exact product title. If I use the text field in addition to the proper category, it finds the matching product(s). If I then remove the category filter, I’m back in the shop, as if no product was found.

Any idea how what could be wrong? Does the text filter option have known issues?