SPD-SX phones channel leaks to master channel

[teisvh]

Hi

I'm encountering a strange problem with my SPD-SX drumpad. The sounds that are being played in the headphones leaks to master channel, most noticeable when I'm using the metronome. So far I have discovered the following through troubleshooting: The sound only leaks to the master channel when there is a cable plugged into the phones output. If this cable is mono the sound will leak immediately. However, if I convert it to a stereo minijack, then it only leaks when that cable is plugged into something, that being headphones or something else.
I just updated the firmware without results.

Does anybody know a fix for this? It would be highly appreciated!

Cheers
Teis