I actually came here to post about this, because I've been experiencing the same issue for a similar length of time. It only occurs on the mobile app for me, too. Both the desktop app and the web player update regularly as expected. If I search for "Your Daily Mix" on the mobile app, it shows the updated mixes that appear on the desktop/web players, but they can't be opened (they react to me pressing them, but they don't open the playlist). I managed to get the playlists to refresh once or twice by doing the following:
Opening the mobile and desktop apps at the same time
Starting a song from the end of the first playlist on the desktop app (first playlist because it only seemed to update playlists that are greater than or equal to the one you play from when this worked one of the few times)
On the mobile app, choose to start playing from the mobile device
On the desktop app, start the new last song in the playlist (the playlist should have expanded after step 2) (the mobile device should still be the output device)
Repeat step 4 a bunch more times (the playlist stopped loading more songs after I did it 8 times)
The next day, your daily mix might be updated.
The usual steps of clean-installing the app, forcing all devices to log out, etc, didn't seem to help.