On iPhone, occurring for last several versions of the OS, the visual track visual disappears.
The bug is consistently related to being used in landscape mode, hitting auto lock limit or manually locking during playback, then unlocking phone in portrait mode.
Here is a specific flow where I can reproduce.
Classic Mode in landscape
Two tracks playing from iTunes library.
Both tracks looped
Auto lock occurs
Phone is unlocked in portrait
App is in protrait
Switch to landscape
Track visual is black