Thanks for reaching out to us about this!
We can confirm that this is currently the expected behaviour of the app: Daily Mixes are now fixed playlists of 50 songs which update the more you listen to them (see here for more). The mobile app has the added functionality of being able to adjust your preferences.
If you'd like to bring this functionality back to the desktop and web player apps, make sure to add your vote to this relevant idea in the Idea Exchange - it's the best way of providing your feedback on specific features!
For more info on how your feedback reaches Spotify via the Idea Exchange, make sure to check out to this Spotify Answer.