We've discussed this Idea internally and have decided to close it.
The reason for this is that if a full screen cover art is shown on the lockscreen has always been dependent on how the Android rom on the particular phone was configured to interact with media playback. This can vary from manufacturer to manufacturer, from one android version to another and from one phone model to the other.
As it stands, the newest Android development is that no full-screen album art will be shown. Due to these changes and the people who received these android updates, we actually started seeing requests for the exact opposite: to have full screen cover art shown on the lockscreen.
As such, we'd like to direct your attention to this idea about an available toggle in the settings that controls the behavior and ultimately makes Spotify interact with the pre-set Android setting. If you feel this is something you want to support, please go give that idea a like.