One-time iOS alarms don’t seem to work right in the future. The problem is with non-recurring alarms set from within the Clock application. For the bug to show itself, your iOS device must tick over from 11:59 p.m. on December 31, 2010 to 12:00 a.m., for example. The workaround for this is easy: set your alarm for New Years Day to be an every Saturday alarm. According to a 9to5mac report, this bug lasts through January 3, then fixes itself.”]
Source: https://www.csoonline.com/article/2126481/ios-bug-prevents-new-year-s-alarms.html