The DDJ itself has no audio device, so the output depends entirely on another audio interface and whether it's configured for multiple outputs (discrete out for master / headphones), or a split output (left audio master, right audio headphones). If the configuration has changed either on the device or in the rekordbox audio settings, that could affect the output. Likewise, if you're using a PC, the audio enhancements or spatial effects may be causing signal crossover between channels. Please check those as well.