A new bug has been discovered in iTunes 12.4 which causes playback issues with Apple Music tracks shorter than a minute or 60 seconds. The root cause of the problem has been attributed to a buffering bug that stalls the streaming for the next track if the current one being played is shorter than 60 seconds.
In other words, iTunes will stall waiting for the next song in the queue to finish downloading, although it has never even started streaming or buffering completely. The bug has already been reported to Apple while the company is yet to acknowledge or release a fix for this problem.
Meanwhile, a member of the MacRumors forum has successfully reproduced the issue using a network-activity monitoring tool called 'fswatch' and identified the root cause of the problem:
It looks like the way Apple Music handles streaming is when the current song is a minute from the end, iTunes signals the next track in the queue to start downloading so that it's ready to play when the current song is over. However, when the song is less than a minute long the next song's download is never initiated, apparently because some "one minute remaining" event is never triggered! This means the app just sits waiting for a download to finish that has in fact never started.
Folks at MacRumors have successfully reproduced the issue on Macs running OS X 10.11.5 and iTunes 12.4.1 while the bug is conspicuously absent when playing locally stored tracks and albums. It is further confirmed that the bug only affects streamed songs via Apple Music. However, the issue is not reproducible in macOS Sierra or iOS 9.3.2.
Check out the MacRumors' demo video depicting Apple Music playback bug on iTunes 12.4.1 in action (below):