Music app doesn't show audiobooks anymore

I used to listen to podcasts, audiobooks, and other recordings on my iPhone, but starting yesterday, the categories in the Music app changed so that I can't access these files anymore.

I tried to open them with other apps (Podcasts, Audible) and it didn't work either.

(to be clear: I can listen to audobooks from Audible in the Audible app; I can also listen to podcasts that I subscribed to in the Podcasts app; but if I download single episodes of podcasts straight from the websites or have other audio files that I declare as audiobooks in iTunes on my Mac and that show up there under audiobooks, there are the ones that don't show up anymore -- though when I connect my phone to the Mac and check from there it shows that the files are on the phone (though they are inaccessible via Spotlight too)).

Here a screen of what my Music app looks like now, the category "Audiobooks" just disappeared: