Headphone Jack Not Disengaging Speakers?

I have a NAD C 326 BEE integrated amp. When I put my headphones into the jack the speakers still play. 
I don't think this is normal, am I not doing something right? I know that there is no speaker output defeat switch so I think this may be a design flaw or it needs repair.  
I do have the integrated passing through to a tube amp so I can simple just not turn on the amp for headphone listening, however I still find it annoying. Also if I decide to sell it I want to be sure in in proper working order. Any thoughts would be welcome.
That's design flow and it's annoying. The only way to get around it is to unplug speakers from amp. The output stage will still be loaded with headphones so your amp will be allright.

According the owner’s manual for that unit, the speaker outputs are supposed to switch off when the headphone plug is inserted.

I would turn the unit off and gently mate the headphone plug with the jack. Make sure it is fully inserted because this is what breaks the circuit to the speaker outputs. Perhaps rotate it. Do this several times.

Then after enjoying a cigarette, turn the volume down, turn the unit on, and try it again. On my older Rotel pre, some gunk and dust prevented getting a good contact.
Thank you both for your reposes and suggestions. I will definitely give that I try!