Wow. The saloon sure is heating up!
I worked as a repair technician for over 40 years. Hadn't seen a product yet that did't have at least a quirk or two. Made a lot of money fixing or even modding them.
The big dog brands put out a new model every year and seemingly never look back at what may have been wrong with a previous year model's feature or two. I even saw where some of these quirks were carried on to the next gen model.

I mentioned owning both of these processors. But I never tried the headphone output on the 975. Being in my main theater, I listen through main speakers 100% of the time. So far, the same is said for my 976.
But, this is my point. The headphone feature on an AV processor to me, is an afterthought feature. One that's added to accommodate a small percentage of users. Perhaps their bedroom processor, or a secondary room.
I could be wrong, but how many actually do use their systems with headphones?
I'm sure that is one feature that Outlaw really didn't spend a lot of time on developing.
I love my 976. Haven't had any issues, other than the periodic HDMI port cleaning. Actually, I've only cleaned the 976 ports once since I've had it in the system. I needed to clean the ports on my 975 several times in the 4 years I owned that one. But that would be true for any HDMI connected devices.
As far as quirkiness goes, the 976, for me, has really been great. Of course I use it in a more traditional system, predominately for movie watching and music listening through my main speakers.

Muting circuits can be 100% mute. Or, they can be a percentage less than 100%. I'm thinking, Ben is telling you the 975 and the 976 isn't muting the speakers 100% for the headphone function. Could it be a lesser percent mute on the 975? Sure.
This thread is 3 pages long now, not many have chimed in. So maybe I'm right. Not many use the headphone feature.