I have seen the canvas feature on the iPhone app change many times. First (#1) it was a full screen visualiser, but slightly hidden by the dark controls overlay. Then when you tapped the screen the dark controls would remove to the nice bright full screen visualiser.
Next it was updated (#2) so the two choices/taps were a full screen with dark overlay, or the artist square album cover (regular view). Which wasn’t great as you couldn’t get the above bright full screen visualiser anymore.
It was then updated again back to (#1), so I was happy again!
Now in the latest update, 15/04/2020, it is back to (#2) version. I think if artists are paying for a full screen visualiser add on for each song, we should be able to select to view it full screen.
Perhaps add another toggle option under Settings>Playback>Canvas?