[Update #2: Apple appears to be resolving this issue.]
[Update: It seems that the issue is affecting more than just Warner Bros movies. Users report that 4K titles from Columbia, MGM, 20th Century Fox, and others have also reverted to HD. This suggests it’s likely a bug on Apple’s part.]
Over the last several days, many Warner Bros movies on iTunes have reverted from 4K to simply HD on iTunes. This includes titles that customers have already purchased. At this point, it’s not explicitly clear whether this is an issue on Apple’s part or Warner’s.