Oh that is bad. Any firmware Updates for your Denon x1300w? And you can try disable all OSD like showing volume when you change it or other picture changing features of the denon, so that is does passthrough for the picture and don't touch it.
My x4500h has a setting in HDMI to disable all picture changing features. I can look for the exact option when I am home.
On my side my NUCP7JYH is connected to a Asus screen through HDMI (only I have HDMI "multiplexer").
I have an amplifier but it's a Tangent Ampster BT connected through optical so I think it's not related...
Before buying the NUC I used an old Zotac Zbox AD-06 and never had any issue like that with it...