That's not unusual, really. For instance, the manager might have been replaced. He might have had a fairly easy-going manager who was non-confrontational, but the new manager is more of a "by the book" guy. Or, the manager might have chosen to overlook it so long as there were no complaints, but perhaps another employee brought it to management attention, so action had to be taken. Or, perhaps corporate sent out a "dress code reminder/reinforcement" notice, and that spurred management to say they had overlooked it but could no longer do so.

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