Hi NomoSony,
That is apparently normal behavior with Yamaha receivers. I've got an HTR-5990 that does the same thing and several other Yamaha owners who post here have mentioned the same behavior.
One option you may be able to take advantage of is the fact that this doesn't effect the switched outlet on the back of the receiver.
I AM NOT SAYING TO PLUG THE AMP INTO THE RECEIVER...
But, if you have a power center with the option to plug a triggerline into the switched outlet, that will work and isn't effected by the "multi ch" trigger. I'm using a Monster HTS3500 with that functionality.
Not the best news I'm sure, but it could be much worse.