This problem occurs when the USB device contains files that are not supported. Delete the unsupported, unplayable files from the USB device.
When the system encounters a file that is not supported, the song file is skipped, and the next song is played. If a song has been skipped, then the number of songs recognized by the system and the number of songs that are saved to the USB device are different. For this reason, when you press the prev/next button, the same song or an earlier song may play.