My Denon AVR-X4400H used to do that for the fist year or so. Others Denon owners reported similar behavior on other models as well. If I remember right, by turning everything that has to do with CEC, even ARC (no, not Anthem's ARC, wish they didn't use that term) and it seemed to have reduced that strange occurrence a lot but still happening. It eventually stopped after may be 2,3 years and many FW update. The FW update log never mentioned any such fix, but I bet they did know the issue and fixed it quietly.
Hopefully, if it is for similar reasons like D+M's, Yamaha will fix it too via FW updates. In this case, you seem to be saying it didn't happen until recently, so it may be something totally different. Have you done anything different recently, such as hooked up something that was not there before, or change some settings related to HDME, CEC etc.?